Evaluation of 15N marked urea in the laying hen. 4. Incorporation of 15N in blood, its fractions and follicles in various developmental stages

In order to study the utilization of urea in poultry, 3 colostomized laying hybrids were orally supplied with a traditional ration supplemented with 1% 15N'-labelled urea with a 15N excess (15N') of 96.06 atom-% over a period of 6 days. After another 2 days on which the hens received the same ration with unlabelled urea, they were butchered. The atom-% 15N' of the blood on an average of the 3 hens was 0.64, of the plasma 1.40 and of the corpuscles 0.47. The TCA-soluble fraction of the blood had an average 15N' of 1.14 atom-%; the 15N amount is 9.7% of the total amount of 15N in the blood. The amount of 15N' in the urea in the blood was 6.8 atom-%. This shows that the absorbed urea is decomposed very slowly. The quota of 15N' in the basic amino acids from the total 15N' of the blood plasma is only 0.3% and that of the corpuscles 2.2%. The average 15N' of the mature follicles is 2.39 atom-% whereas the smallest and the remaining ovary contain 1.12 atom-%. The labelling level of lysine in mature egg cells was, in contrast to this, only 0.08 atom-% 15N' and in infantile follicles 0.04 atom-% 15N'. 1% of the 15N' quota is in the follicles and the remaining ovary. Of the basic amino acids, histidine is most strongly labelled. The as a whole lower incorporation of the 15N from urea into the basic amino acids shows that the nitrogen of this compound can be used for the synthesis of the essential amino acids to a low degree only.     

Dynamics of amino acid and protein metabolism in laying hens after the administration of 15N-labeled wheat protein. 2. 15N excretion with nitrogen and basic amino acids of the feces

Over 4 days 12 colostomized laying hens received together with a commercial ration labelled wheat with a 15N excess (15N') of 14.37 atom-%. The labelling of the basic amino acids amounted to 13.58 atom-% for lysine, to 14.38 atom-% for histidine and to 13.63 atom-% 15N' for arginine. 3 animals each were butchered 12 h, 36 h, 60 h and 108 h resp. after the last application of 15N. The heavy nitrogen in the total N and in the N fraction of non-protein origin as well as in the basic amino acids in faeces was daily determined for the individual hens in the total experimental period. On average the crude protein of faeces contained 5.45 % lysine, 2.32% histidine histidine and 3.68% arginine: the protein of faeces correspondingly contained 5.43% lysine, 2.32% histidine and 4.07% arginine. The quota of TCA soluble N in the total N of faeces amounts to one third on the 3rd und 4th days of the experiment and that of 15N' to 28%. The average atom-% 15N' of the protein fraction is 3.48 atom-% 15N' and that of the non-protein N fraction of faeces 2.93 atom-% 15N'. The apparent digestibility and that of the non-protein N fraction of faeces 2.93 atom-% 15N'. The apparent digestibility of the 14N of the ration on average amounts to 82.8% and that of the wheat 15N' to 87.5%. The average quota of the basic amino acids in the protein compounds of faeces amounts to 70.9% for lysine 15N', 73.7% for histidine 15N' and 70.3% for arginine 15N'. The digestibility of the 15N labelled amino acids amounts to 80.4% for lysine, 90.8% for histidine and 90.2% for arginine.     

Utilization of 15N-labeled urea in laying hens. 7. 15N-incorporation into the amino acids of different muscle types

3 colostomized laying hybrids received 1% 15N labelled urea with 96.06 atom-% 15N excess (15N') with a commercial ration over a period of 6 days. After the application of the same ration with unlabelled urea on the following 2 days the animals were butchered. In the muscles of the breast, the leg and the heart, the labelling of total nitrogen and the incorporation of urea 15N' into 15 amino acids of the 3 different kinds of muscles were ascertained. On average, significant differences could be ascertained between the atom-% 15N of the muscles of the skeleton and those of the heart. The 15N' of the breast and leg muscles was 0.25 and 0.34 atom-% resp.; that of the cardial proteins 0.71 atom-% 15N'. The incorporation of urea 15N into the basic amino acids is low and varies both between the kinds of muscles and between the amino acids. On average the highest level of labelling was found among the essential amino acids valine, isoleucine and leucine; the average atom-% 15N' for the muscles of the breast is 0.13, of the leg 0.17, and of the heart 0.27; the 15N' quota of branched chain amino acids in the total 15N' of the respective muscle is accordingly 6.0%, 5.0% and 4.5%. The non-essential amino acids, particularly glutamic acid, are more highly labelled in the muscles than the essential ones. A 15N' for glutamic acid of 0.24 atom-% in the breast muscles, of 0.27 atom-% in those of the legs and of 0.64 atom-% in the heart muscle could be detected. The average quota of the 15N' of these acid amino acids in the 15N' for breast, leg and heart muscles is 7.4, 6.2 and 6.7 resp. The quota of the 15N' in the 6 non-essential amino acids in the total 15N' in all 3 kinds of muscles is approximately two thirds and in the 9 essential ones one third of the total 15N'. Although the results show that there is a certain incorporation of 15N' from urea into the amino acids of the muscle proteins, their contribution to meeting the demands is to be considered irrelevant.     

Utilization of N15-labelled urea in the laying hen. 6. Incorporation in the liver and kidneys

In an experiment 3 colostomized laying hybrids received a normal ration containing 1% 15N labelled urea with 96.06% atom-% 15N excess (15N') over six days. Subsequently the same ration with unlabelled urea was given over 2 days, after which the animals were butchered. In the kidneys the 15N' amounted to 1.1 atom-% and 1.8 atom-% in the liver. The TCA soluble N fraction and the ammonia were more highly labelled than the total N. Lysine, histidine and arginine were lowly labelled in the kidneys. This also applies to the liver with the exception of histidine. In the branch-chained and aromatic amino acids of the liver the 15N' was between 0.2 and 0.3 atom-%. The highest labelling of non-essential amino acids was found in glutamic acid with 0.9 atom-% 15N' and aspartic acid with 1.1 atom-% 15N'. The evaluation of the amino acid in the liver showed that the 6 non-essential amino acids account for two thirds of the total amino acid 15N' whereas the 9 essential ones account for one third of the amino acid 15N' only.     

Dynamics of amino acid and protein metabolism of laying hens after the administration of 15N-labeled wheat protein. 3. Incorporation of 15N into egg shell, egg white and egg yolk

12 colostomized laying hybrids received a ration meeting their requirement of 15N labelled wheat with a 15N excess (15N') of 14.37 atom-% over 4 days. The 15N' of the total ration amounted to 4.47 atom-%. Each hen consumed 135 mg 15N' per day. On another 4 days the same rations with non labelled wheat were fed. The 12 hens laid 56 eggs during the 8 days of the experiment. They were divided into egg shell, white and yolk of egg. In addition, the protein of the white and yolk of egg was precipitated with trichloric acetic acid (TCA) and the nitrogen in these fractions was determined. On average of the 56 eggs, the N quota in the egg shell was 5.3%, in the white of egg 49.1% and in the yolk 45.6%. The atom-% 15N' in the shells of the eggs laid on the first day of the experiment was on average 0.21, whereas only 0.03 and 0.02 atom-% 15N' resp. could be detected in the white and yolks of the eggs. On the first day after the last 15N application the atom-% 15N' in the egg shell and the white of egg was highest and amounted to 2.33 and 2.43 atom-% resp. The highest value of 1.83 atom-% 15N' in the yolk was ascertained 3 days after the last 15N intake. The mean quota of TCA-precipitable N in the white of egg is 97.6% and in the yolk 94.4% of the respective total N. The atom-% 15N' in the non-protein N-compounds was higher than in the protein fractions.     

Utilization of 15N-labeled urea in laying hens. 9. Bones, feathers and 15N-incorporation in the rest of the body

For studying the incorporation of the 15N labelled urea into individual organs and tissues 3 colostomized laying hens were butchered after they had received 1% urea (96.06 atom-% 15N excess) with a high quality ration over a period of six days and after receiving conventional urea for another two days. Nitrogen and atom-% 15N excess (15N') were determined in the bones, the feathers and the remaining body (skin, lungs and windpipe, head with comb and wattle, lower leg without bones and with skin, pancreas and fatty tissue). In the remaining body the atom-% 15N' was determined in 15 amino acids. The labelling in the remaining body and the bones was approximately the same and averaged 0.37 atom-% 15N'. A significantly lower relative frequency could be detected in the feathers. The lysine of the remaining body contained only 0.04 atom-% 15N', tyrosine 0.06, histidine and arginine 0.07. The phenylalanine and proline molecules were labelled with 0.11 atom-% 15N'. Most 15N' was incorporated in serine and glutamic acid with over 0.30 atom-%. In the six non-essential amino acids out of the 15 amino acids studied, 48.6 of the non isotopic nitrogen of the total N of the remaining body and 70.7% of the isotopic nitrogen of total 15N' could be detected. Consequently the urea-N is mainly used for the synthesis of the non-essential amino acids, with its utilization being very low.     

Utilization of 15N-labelled urea by laying hens. 1. Survey of the literature, 15N-excretion in feces and urine

Three colostomated leghorn hybrids with an average laying performance of 75% received a ration with 17.7% crude protein and an energy content of 519 energetic feed units for hens per kg mixed feed over a period of 8 days. In the first six days of the experiment the 1%-supplement of urea to the ration was labelled. Its atom-% 15N excess (15N') amounted to 96.06%. During the last two days the urea supplement was not labelled. The total N, trichloracetic acid (TCA)-soluble N and the ammonia N were determined in the feces samples collected daily. In the urine samples collected daily the total N, urea N and ammonia N per hen were determined as well. In all samples the atom-% 15N excess (15N') was measured. The percentage of 14N in feces of the 14N dose was, on an average of the three hens, 21.3% and the analogous quota of 15N' 4.6%. The quota of ammonia 14N of the total 14N in feces had an average of 2.5%, the corresponding 15N' quota was 10.1%. The atom-% 15N' of the urea N in urine was considerably above that of the total urine N and had a maximum of more than 50%. The quota of urine 14N of the 14N taken in had an average of 44.4%, and the corresponding 15N' quota was 56.9%. On an average of the three hens, 61.6% of the 15N' were excreted in feces and urine during the 8-day test period.     

Utilization of N15-labelled urea in the laying hen. 5. N15-labelling of the content and tissue of separate parts of the gastrointestinal tract

In the series of experiments with labelled urea three colostomized laying hybrids were butchered after a six-day application of 1% urea with 96.06 atom-% 15N excess (15N') in the ration and another 2 days with a supplement of 1% unlabelled urea. Out of the individual samples from crop, gizzard, small intestine, caecum and rectum, the content of the small intestine and the caecum showed the highest labelling with greater than 1 atom-% 15N'. The TCA soluble fraction of the content of the gizzard was more highly and that of the intestines less labelled than the total nitrogen. The tissue of the gizzard is distinctly less labelled than the "omasum system" and the small intestine. The atom-% 15N' of the oesophagus with crop and glandular stomach largely showed agreement in the individual hens with that of intestinal tissue and ranged between 0.71 and 0.89 atom-%. 2% of the 15N' supplemented with the urea could be recovered in the content and the tissue of the gastro-intestinal tract.     

Urea utilization in growing lambs. 4. N balances with unprocessed rations

N balance experiments were carried out with lambs of the ages of 8, 12 and 15 weeks fed with wheat rations with and without 2% urea supplement (N 1 and N 2) as well as with 3% urea and 20% straw (N 3) or with a soya bean meal supplement (N 4). There were no significant differences in the digestibility of the crude nutrients and in per cent of N retention between the individual ages. The straw supplement decreased the digestibility of organic matter, crude protein, crude fibre and NfE. The supplements of soya bean meal or urea increased the crude protein content in comparison to the wheat ration without supplements by 6% in the dry matter and resulted in N intakes 55 ... 60% higher and in 23 ... 38% higher N retention, which was, however, lower in relation to N intake. There were no significant differences with regard to N retention between N 2, N 3 and N 4. Consequently urea supplement to the feed mixture with 14% native crude protein resulted in increased N retention, which was not lower than with a soya bean meal supplement.     

Urea utilization in growing lambs. 7. NPN- and pure protein-N-utilization at various ages

The utilization quota of NPN and pure feed protein for body protein synthesis was calculated on the basis of N balance experiments with 15N labelled urea with the help of model concepts of a 3-pool model and its mathematical usage. In lambs weighing 13 kg, the efficiency of amino acid and nucleic acid synthesis in the non-amino acid N pool was 64%. This results in a total utilization quota for NPN and pure protein in the ration of 40% and 60%, resp. Lambs weighing 27 kg showed an efficiency in amino acid and nucleic acid synthesis in the non-AA N pool of 77% and in the AA N pool of 60%. The total utilization quota of NPN was 47% and that of pure protein 56%. The pure protein in the ration was thus about twice as well utilized for total protein synthesis and for protein synthesis for crude protein retention as the NPN compounds in the ration.     

Dynamics of amino acid and protein metabolism of laying hens after administration of 15N-labeled wheat protein. 6. 15N incorporation into the contents and tissue of segments of the gastrointestinal tract and into the pancreas

12 colostomized laying hens received, together with a conventional feed ration, 15N labelled wheat with a 15N excess (15N') of 14.37 atom-% over 4 days. 3 animals each were butchered after 12 h, 36 h, 60 h and 108 h after the last 15N' application and, apart from various organs, the contents and the tissue of the gastro-intestinal tract of each hen was divided into 3 fractions. TCA precipitation was carried out with the contents and the tissue of the 3 fractions. Nitrogen and its atom-% 15N' were determined in the supernatant and the precipitate. The 15N' amount in the contents of the crop and the stomachs, the small and large intestines is still considerable 12 h after the last 15N wheat feeding and still clearly detectable 108 h after it. The TCA precipitable amounts of 14N and 15N' of the contents of crop and stomach and that of the small intestine agree well; they are 75% and 50% resp. of the total N. The amount of atom-% 15N' of the contents of the small and large intestines remains the same up to 36 h after the last 15N' application and is higher at the following measuring points in the contents of the large intestine. A close correlation could be ascertained between the atom-% 15N' in the contents and tissue of the small and large intestines. The TCA soluble N quotas of both 14N and 15N' in the pancreas are higher than 50%.     

First results on the incorporation and excretion of 15N from orally administered urea in lactating pony mares

Two lactating pony mares were given oral offers of 20 g 15N urea [95 atom-% 15N-excess (15N')] on 6 subsequent days. About 80% of the consumed 15N' were excreted via urine and faeces, but only about 2% via milk. The 15N' secreted via milk-lysine only amounted to 0.04% of the 15N' intake. The recovery was about 90% in each case. Tissues with active metabolism had an unexpectedly high labelling (greater than 0.3 atom-% 15N'). The low extent of the conversion of oral urea N into milk-lysine speaks against an essential participation of the enteral synthesis in meeting the amino acid requirement of lactating mares. It was already concluded from this results that the determination of the amino acid requirement will be necessary for this group of performance.     

15N-labeled lysine in colostomized laying hens. 5. 15N incorporation into blood fractions and the gastrointestinal tract

In the metabolism experiment three colostomized laying hens received, together with a commercial ration of 120 g, 0,2% 15N-labelled L-lysine with an atom-% 15N-excess (15N') of 48%; subsequently the same ration was fed over a period of 4 days with 0.2% unlabelled L-lysine. After the end of the experiment the hens were slaughtered. The atom-% 15N' was determined in total, in the lysine, histidine and arginine N in the corpuscles, the plasma, the NPN-fraction of the blood in the stomachs, the small intestine, the caecum and the rectum. 15N' in the corpuscles was 0.11 atom-%, in the blood plasma 0.17 atom-%, in the NPN-fraction of the blood 0.09 atom-%, in the tissue of the gastro-intestinal tract 0.11 atom-% and in its contents 0,12 atom-%. On average the blood contained per hen 77.9% lysine-15N', 16.4% arginine-15N' and 5.7% histidine-15N' of the basic amino acid-15N'. For the gastro-intestinal tract 78.7% lysine-15N', 19.0% arginine-15N' and 2.3% histidine-15N' of the 15N' of the basic amino acids were ascertained. In comparison to histidine the alpha-amino-N of lysine is incorporated to a considerably higher degree into arginine. For lysine and arginine the atom-% 15N' in the contents of the gastro-intestinal tract for days after the end of the supplementation of labelled lysine is between 8 and 10 times higher than in the feces of the last day of the experiment. This indicates a considerable secretion of the two amino acids in the gastro-intestinal tract and their re-absorption to a large extent.     

Urea utilization by growing lambs. 5. N-balance with half synthetic rations

In experiment with 14-week-old lambs a combination of semisynthetic rations containing 1.2% or 0.7% resp. native crude protein an intraruminal infusion of urea clearly influenced the biologic processes in the rumen. The molar quotas of butyric and valeric acid in comparison with acetic were high. The lambs could cope with infused amounts of 0.67 g urea/kg body weight if they received the daily amount of 1.35 g/kg body weight in two parts. When more than 1.9 g/kg body weight were applied, feed intake decreased even though the daily amount was distributed over 4 doses. In 10 out of 16 lambs N retention was, in dependence on the urea amounts infused, higher than the N amount retained from native crude protein. The result indicates that urea is utilized by 14-week-old lambs. Growing energy and N intake had a positive influence on the N balance.     

Utilization of 15N-labeled urea in laying hens. 8. 15N-incorporation in the amino acids of the oviduct

3 colostomized laying hybrids received orally with a conventional ration 1% urea with 96.06 atom-% 15N excess (15N'). over a period of 6 days. In the period of the experiment every hen consumed 2.87 g 15N'. After another 2 days, on which they received conventional feed urea, the animals were butchered. 15N' was determined in the total N and in 15 amino acids of the oviduct. Of the 15 amino acids the labelling of glutamic acid, glycine and serine was highest and on average amounted to 0.80, 0.66 and 0.67 atom-% 15N'. In lysine and arginine only 0.10 and 0.11 atom-% 15N' could be detected. The amino acid N with natural isotopic frequency amounted to a quarter for the basic amino acids, a tenth for the branched chain ones and for the non-essential ones (glutamic acid, aspartic acid, serine, glycine, alanine, proline) a third of the total oviduct 14N, The average quota of 15N' is only 3.6%, that of the branched chain amino acids 4.5 and that of the non-essential ones 21.1%. Consequently, the 15N' of the urea is mainly used for the synthesis of the non-essential amino acids of the oviduct.     

Dynamics of amino acid and protein metabolism of laying hens after administration of 15N-labeled wheat protein. 5. Incorporation of 15N into the blood fraction and its amino acids

12 colostomized laying hens which received 15N labelled wheat over 4 days were butchered 12 h, 36 h, 60 h and 108 h (3 animals each) after the last 15N application. The intake of 15N excess (15N') from the wheat amounted to 540 mg 15N' during the application period. The 15N' in the blood plasma decreased after the last 15N' application from 0.76 atom-% to 0.55 atom-% after 108 h, the labelling of the corpuscular components at the same measuring points increased from 0.28 to 0.50 atom-% 15N'. 96.6% of the plasma 15N' and 93.8% of that in the corpuscles is precipitable in trichloric acetic acid. The atom-% 15N' of histidine in the total blood remained unchanged in dependence on the butchering time. The 15N amount in lysine and arginine and that in the non-basic amino acids decreased inconsiderably in the period between 12 h and 108 h after the last 15N' wheat feeding.     

Studies on nitrogen metabolism in the large intestine of ruminants. 5. Metabolism of intra-cecally infused 15N-urea without and with fermentable material in heifers

Six heifers with a live weight of 215, 227 and 238 kg (experiment 1) and 220, 227 and 233 kg, resp. (experiment 2), were supplied with ileocaecal re-entrance cannulae, jugular venous catheters and bladder catheters. The ration consisted of 4 kg maize silage and 4 kg wheat straw pellets per animal per day. Up to 3.5 kg of the straw pellets, consisting of 73% wheat straw, 10% barley, 12% molasses, NPN salts and a mineral mixture, were consumed per animal per day. In a preliminary period 50% of the digesta flow was collected over 12 h/d on 5 consecutive days and stored in a deep-freeze. During the main trial the re-entrance cannula was disrupted and the flowing digesta were quantitatively collected at the end of the ileum; previously collected digesta were supplemented with 15N urea and every hour over 24 h infused into the caecal part of the re-entrance cannula. Between the 24th and 30th hours the digesta were infused without 15N urea supplement. In trial 2 the digesta were also supplemented with partly hydrolysed straw meal between the 1st and 30th hours (approximately 10% straw meal DM related to digesta DM). There were no differences between trials 1 and 2 with regard to the increase of atom-% 15N excess (15N') in the plasma urea. The 15N labelling decrease of the plasma urea N shows that the half-life is 7.9 h in trial 1 and 7.0 h in trial 2. The NH3 nitrogen in faeces was distinctly higher labelled in trial 2 after the supplement of straw meal than in trial 1. The total N in faeces was also twice as highly labelled as in trial 1. Atom-% 15N' in urine was significantly higher in trial 2 than in trial 1 between the 6th and 16th hours after the beginning of 15N urea supplementation. In the decrease curve of atom-% 15N' (after the 26th hour of trial) the values in trial 1 were generally higher than in trial 2. The higher bacterial protein synthesis in the large intestine in trial 2 (after the supplement of partly hydrolysed straw meal) had the effect that 13.6% of the supplemented 15N' was excreted in faeces by the 30th hour of trial, in contrast to this only 4.7% in group 1. Up to the 4th day after the 15N urea infusion these values increased to 16.2 and 6.1%, resp., only.     

Dynamics of amino acid and protein metabolism of laying hens after the administration of 15N-labeled wheat protein. 8. 15N-labeling of nitrogen and 15N incorporation into the amino acids of the liver

Over 4 days 12 colostomized laying hens received, together with the ration, 36 g wheat with 14.37 atom-% 15N excess (15N'), The basic amino acids were nearly equally labelled. Three animals each were butchered after 12 h, 36 h, 60 h, and 108 h after the last 15N' application. Emission spectrometric determination of 15N' in the liver and in the amino acids was carried out. In addition, atom-% 15N' was determined in the free amino acids and the peptides. The labelling in the liver 12 h after the last 15N' application amounted to 1.75 atom-% 15N' and decreased after 108 h to 0.81 atom-% 15N'. The average TCA precipitable 15N' quota in the total 15N' amounted to 81.4% and was nearly identical at all measuring times. The arginine 15N' amount in the liver was twice as high as that of lysine 15N'. In dependence on the period of time after the last 15N' application the decrease in the labelling of the free arginine is considerable in comparison to free lysine. At the first measuring time (12 h) it was 1.69 atom-% 15N' and at the last one (108 h) 0.57 atom-% 15N'. Based on the results of 15N' labelling of the peptides in the liver further, more detailed series of experiments for studies of the peptide metabolism in the liver should be carried out.     

Effect of the content of crude plant protein in the ration on the utilization of urea in dairy cattle. 2. 15N-urea metabolism

The metabolism of 15N-urea in the rations of dairy cows was investigated in dependence on the crude protein content of the rations. With energy concentration remaining unchanged, the rations contained 10.7 (I), 13.7 (II) and 17.1 (III)% plant crude protein and, after the supplementation of 150 g urea per animal and day, a total of 13.8, 16.7 and 20.2% crude protein in the dry matter. The urea was intraruminally infused during the feeding in the morning and the evening. In the morning feeding of each 1st measuring day it was labelled with 27.5 atom-% 15N-excess (15N'). The degree of labelling with 15N' of the N-fraction of rumen fluid, contents of the duodenum, faeces and milk, precipitable with trichloric acetic acid (TCA) decreased with the rising protein level of the ration. This effect was bigger than could be expected considering the low 15N'-quota in the total-N of the ration. In the sequence I ... III, 52.7, 32.2 and 30.6% of the 15N'-amount taken in passed the duodenal re-entrant cannula in TCA-precipitable form within 72 hours after the 15N-application. 33.3, 21.9 and 22.6% were apparently absorbed in the intestines as TCA-precipitable N within 120 h after the 15N'-application. In the same period 31.7, 43.1 and 72.8% of the 15N' taken in were excreted in urine. 12.3, 9.6 and 5.8% of the applied 15N' were found in milk protein. One can conclude that the utilisation of urea-N decreases with the rising level of crude protein in the ration and that, however, urea-N is still biochemically utilised when there is an excess of plant-N in the ration.     

Dynamics of amino acid and protein metabolism in laying hens after the administration of 15N-labeled wheat protein. 10. 15N-labeling of crude protein and amino acids of the oviducts

Over a period of 4 days 12 colostomized laying hens daily received 36 g coarse wheat meal containing 14.37 atom-% 15N excess (15N') together with a conventional ration. After the homogenisation of each oviduct N and 15N' were determined. After the precipitation with TCA the 15N' of the amino acids was analysed in both the precipitate and the supernatant. In addition, the free amino acids and the peptides were determined in the TCA soluble fraction. The atom-% 15N' in the total N and in the non-basic amino acid N showed a parallel decrease; it diminished from 1.75 atom-% 15N' to 0.64. Of the three basic amino acids, lysine shows the lowest labelling at all four measuring points. The quotas of non-basic amino acid 14N and 15N' in the total 14N and 15N' of the oviduct are the same and amount to 53%. In contrast to this, the quota of the 14N of the basic amino acids in the total 14N of the oviduct only amounts to 21.6% and that of 15N' only to 15.4%. The average atom-% 15N' of the free amino acids 12 h after the last 15N application is 1.54 and is considerably above that of the peptides with 1.15 atom-% 15N'. 36 h after the last 15N application the ascertained value of 1.25 is identical in both fractions. The labelling of the free amino acids decreases more quickly than that of the peptides the more time has passed after the last 15N application.