The metabolism of [14C] cymoxanil in the rat

A rat, given a single oral dose of [¹⁴C] cymoxanil, 1-(2-cyano-2-methoxyimino-[2-¹⁴C]-acetyl)-3-ethylurea, eliminated 91% of the radioactivity within 72 h. The urine contained 71%, the faeces 11%, and the expired air about 7% of the radiolabel; no ¹⁴C residue was found in the internal organs. Greater than 70% of the radioactivity in the urine was identified. The major metabolite was characterised as glycine, both free and conjugated, as hippuric acid and phenylaceturic acid [N-(phenylacetyl)-glycine], and probably in the form of polypeptides of low molecular weight. The other metabolites identified included 2-cyano-2-methoxyiminoacetic acid, 2-cyano-2-hydroxyiminoacetic acid and 1-ethylimidazolidine-2, 4, 5-trione. The minor metabolites included succinic acid and 2-oxoglutaric acid which indicated reincorporation of metabolic ¹⁴C. Cymoxanil, as such, was not detected in the urine.     

Metabolism of [14C]parathion and [14C]paraoxon in isolated,perfused rat livers

Perfusion of ¹⁴C-(ring)-parathion or ¹⁴C-(ring)-paraoxon with blood through isolated, intact rat livers resulted in the rapid degradation of these insecticides. Degradation was negligible in the absence of rat liver (controls), thus demonstrating the capacity of the liver per se to effectively degrade these compounds. Of the total radiocarbon recovered after liver perfusion with [¹⁴C]parathion, 33 % could be attributed to unchanged [¹⁴C]parathion (similarly distributed between the liver and the blood) while 67.9 % was degraded to water soluble compounds and 2.5% was converted to organic soluble paraoxon and traces of p-nitrophenol. Nearly all of the [¹⁴C]paraoxon, however, was degraded by the intact rat liver, resulting in water soluble products that amounted to 98.5% of the total radiocarbon recovered. Unexplained losses of radiocarbon with the perfusion apparatus used were lower in the presence of rat liver which degraded the insecticides to more water soluble compounds. The water soluble degradation products produced from [¹⁴C]parathion and [¹⁴C]paraoxon were non-toxic to mosquito larvae (Aedes aegypti L.). These ring-labelled products were found to be conjugated p-nito-phenol. Nearly all of the water soluble radiocarbon was located in the perfused blood, while only small amounts (1.8 to 3.0% of recovered) were excreted via the bile or were associated with the liver tissue (1.3 to 1.8 % of recovered).     

Metabolism of [14C]-2,4-dichlorophenol in edible plants

Several 2,4-dichlorophenoxyacetic acid (2,4-D)-sensitive plants have been modified by genetic engineering with tfdA gene to acquire 2,4-D tolerance. The expression product of this gene degrades 2,4-D to 2,4-dichlorophenol (DCP), which is less phytotoxic but could cause a problem of food safety. After a comparison of 2,4-D and DCP metabolism in transgenic 2,4-D-tolerant and wild cotton (Gossypium hirsutum L.), a direct study of DCP metabolism in edible plants was performed. After petiolar uptake of a [U-phenyl-(14)C]-DCP solution followed by a 48 h water chase, aqueous extracts were analysed by high-performance liquid chromatography. Metabolites were thereafter isolated and their structural identities were determined by enzymatic and chemical hydrolyses and mass spectrometry analyses. The metabolic fate of DCP was equivalent to 2,4-D metabolism in transgenic 2,4-D-tolerant cotton. In addition, DCP metabolism was similar in transgenic and wild cotton. The major terminal metabolites were DCP-saccharide conjugates in all species, essentially DCP-(6-O-malonyl)-glucoside or its precursor DCP-glucose. The significance of this metabolic pathway with regard to food safety is discussed.     

Degradation of [14C]Terbuthylazine and [14C]Atrazine in Laboratory Soil Microcosms

The degradation and formation of major chlorinated metabolites of terbuthylazine and atrazine in three soils (loamy clay, calcareous clay and high clay) were studied in laboratory experiments using molecules labelled with ¹⁴C on the s-triazine ring. Soil microcosms were treated with the equivalent of 1 kg ha^-1 of herbicide and incubated in the dark for 45 days at 20(±1)°C. The quantity of [¹⁴C]carbon dioxide evolved in the soils treated with atrazine was negligible and could not be attributed to mineralization of the parent molecule. The mineralization of terbuthylazine accounted for 0·9–1·2% of the initial radioactivity. In the soils studied, the extrapolated half-lives varied from 88 to 116 days for terbuthylazine and 66 to 105 days for atrazine, with no significant differences for the three soils and the two molecules. The deethyl metabolites of the two s-triazines and the deisopropyl-atrazine metabolite appeared during the incubation in the three soils. The completely dealkylated metabolite was not detected in any of the soils. After 45 days of incubation, the non-extractable soil residues for the high clay, loamy clay and calcareous clay soils represented for terbuthylazine, 33·5, 38·3 and 43·1% and for atrazine, 19·8, 20·8 and 22·3% of the initial radioactivity. © 1997 SCI.     

Metabolism of [C]vernolate in soybeans

Penetration and metabolism of [¹⁴C]vernolate in soybean [Glycine max (L.) Merr. var Ransom] pods and seeds were measured 0, 1, 4, 24, 48, or 72 hr after treatment which occurred at 40 days after flowering. Total ¹⁴C recovery decreased ca. 50% within 4 hr and the loss of ¹⁴C was considered to be a measure of volatility. Total nonpolar extractants decreased in a logarithmic pattern which approached 10% of total ¹⁴C recovered within 24–48 hr. Total polar extractants increased in a logarithmic pattern to a maximum of 90% of total ¹⁴C recovered within 24 hr. Seed nonpolar extractants never exceeded 2% of the total ¹⁴C recovered while pod nonpolar extractants consisted of vernolate plus an unidentified component that did not thin-layer chromatograph (TLC) as the sulfone or sulfoxide. Pod polar extractants increased with time to ca. 75% of the total ¹⁴C recovered (24–48 hr) and decreased to ca. 58% at 72 hr after treatment. Seed polar extractants averaged ca. 10% of total ¹⁴C recovered for the first 48 hr after treatment and then increased to 30% of total ¹⁴C recovered. Thus, [¹⁴C]vernolate per se concentration decreased to <1% of applied material within 72 hr through volatilization and degradation of nonpolar extractants to polar products. Polar metabolites showed two major patterns of vernolate detoxification. One detoxification system produced ¹⁴C-metabolites whose R_f's were equivalent to that reported in corn (Zea mays L.) [J. P. Hubbell and J. E. Casida, [J. Agric. Food Chem. 25, 404 (1977)] and accounted for <30% of the pod polar extractants. A second detoxification system was most prevalent in soybean pod and seed tissues and resulted in very rapid modification of vernolate with an unidentified product that was 85% of the extracted ¹⁴C within 4 hr after treatment and which decreased in concentration with time. Therefore, unexplained vernolate detoxification system(s) exist in soybean pod and seed.     

Studies on the mode of action of asulam,aminotriazole and glyphosate in Equisetum arvense L. (field horsetail). II: The metabolism of [14C]asulam, [14C]aminotriazole and [14C]glyphosate

The metabolism of [¹⁴C]asulam (methyl 4-aminophenylsulphonylcarbamate), [¹⁴C] aminotriazole (1H-1,2,4-triazol-3-ylamine) and [¹⁴C]glyphosate (N-(phosphonomethyl)glycine) were assessed in Equisetum arvense L. (field horsetail). Following application of the test herbicides (4mg?0.3 °Ci herbicide/shoot) to the shoots of 2-year-old pot-grown plants, the total recovery of ¹⁴C-label after 1 week and 8 weeks was high for all three herbicides (>80-0% of applied radioactivity). Asulam was persistent (>69-7% of recovered radioactivity) in both shoots and rhizomes. Sulphanilamide, a hydrolysis product of asulam, accounted for the remainder of the recovered radioactivity. Aminotriazole showed evidence of conjugation in shoots and rhizomes. The principal ¹⁴C-labelled component in shoots was composed of high proportions of aminotriazole (>76-3%) together with the metabolites: X (ninhydrin positive), β-(3-amino-1,2,4-triazolyl-1-)α-alanine, Y (diazotization positive) and various unidentified compounds. Rhizomes generally contained lower proportions of intact aminotriazole (>59.4%) together with the metabolites X,Y and unidentified compounds. The proportion of aminotriazole did not decrease with time in shoots or rhizomes; however, the ratio of metabolite X: Y moved in favour of Y as the interval after treatment increased. Glyphosate was extensively metabolised in shoots and rhizomes to yield aminomethylphosphonic acid (AMPA) and various unidentified compounds. Differential metabolism appears to be one of the factors which may govern the persistence and toxicity of the test herbicides in E. arvense.     

Fate of [14C]daminozide and [14C]maleic hydrazide in American elm (Ulmus americana L.)

Radiolabelled daminozide and maleic hydrazide (MH) were injected into American elm seedlings, kept in nutrient solution, to determine their translocation pattern and metabolic fate. Both compounds were rapidly translocated to all parts of the plant. After 21 days, 13% of the applied ¹⁴C was exuded into the nutrient solution from the roots of the plants treated with MH. Using gel-filtration and thin-layer chromatographic techniques, it was determined that daminozide did not form any metabolite, and that MH was converted into a MH-sugar complex. A significant amount of ¹⁴C was unextractable from the plant tissue.     

Metabolism of [14C]vernolate in soybeans

Penetration and metabolism of [¹⁴C]vernolate in soybean [Glycine max (L.) Merr. var Ransom] pods and seeds were measured 0, 1, 4, 24, 48, or 72 hr after treatment which occurred at 40 days after flowering. Total ¹⁴C recovery decreased ca. 50% within 4 hr and the loss of ¹⁴C was considered to be a measure of volatility. Total nonpolar extractants decreased in a logarithmic pattern which approached 10% of total ¹⁴C recovered within 24–48 hr. Total polar extractants increased in a logarithmic pattern to a maximum of 90% of total ¹⁴C recovered within 24 hr. Seed nonpolar extractants never exceeded 2% of the total ¹⁴C recovered while pod nonpolar extractants consisted of vernolate plus an unidentified component that did not thin-layer chromatograph (TLC) as the sulfone or sulfoxide. Pod polar extractants increased with time to ca. 75% of the total ¹⁴C recovered (24–48 hr) and decreased to ca. 58% at 72 hr after treatment. Seed polar extractants averaged ca. 10% of total ¹⁴C recovered for the first 48 hr after treatment and then increased to 30% of total ¹⁴C recovered. Thus, [¹⁴C]vernolate per se concentration decreased to <1% of applied material within 72 hr through volatilization and degradation of nonpolar extractants to polar products. Polar metabolites showed two major patterns of vernolate detoxification. One detoxification system produced ¹⁴C-metabolites whose R_f's were equivalent to that reported in corn (Zea mays L.) [J. P. Hubbell and J. E. Casida, [J. Agric. Food Chem. 25, 404 (1977)] and accounted for <30% of the pod polar extractants. A second detoxification system was most prevalent in soybean pod and seed tissues and resulted in very rapid modification of vernolate with an unidentified product that was 85% of the extracted ¹⁴C within 4 hr after treatment and which decreased in concentration with time. Therefore, unexplained vernolate detoxification system(s) exist in soybean pod and seed.     

Metabolism of [2,5-14c]piperazine in barley plants

The shoots of barley plants root-treated with [2,5-¹⁴C]piperazine were analysed 30 days after treatment. Methanol extraction left a solid residue which contained 31.9% of ¹⁴C (all percentages refer to the total of ¹⁴C incorporated into the shoots); further extraction with acidified methanol and dimethyl sulphoxide dissolved respectively 3.2% and 5.8% of ¹⁴C. The initial methanol extract contained radioactive piperazine (16.8%), iminodiacetic acid (8.6%), glycine (15.4%), oxalic acid (7.2%), and un-identified compounds (20.1%). In barley, piperazine is the product of the most advanced metabolism so far identified of the fungicide triforine, 1,4-bis(2,2,2-trichloro-1-formamidoethyl)piperazine; the results obtained here show that piperazine is certainly not the end-product of the metabolism of triforine in barley.     

Binding of [14C]DDT and [14C]dicyclohexylcarbodi-imide to a lipoprotein of the membrane sector of mitochondrial ATpase

Intact mitochondria, isolated from red coxal muscle of the American cockroach (Periplaneta americana L.), were incubated in the presence of 1,1,1-trichloro-2,2-bis(4-chloro[¹⁴C]phenyl)ethane ([¹⁴C]DDT) to isolate a suspected binding site for DDT in the membrane sector of the mitochondrial ATPase. The requirements for the binding of DDT were compared with those for the binding of dicyclohexyl[¹⁴C]carbodi-imide([¹⁴C]DCCD), a potent inhibitory probe of mitochondrial ATPase activity. [¹⁴C]DDT appeared to bind to a proteolipid of the membrane sector, which also binds [¹⁴C]DCCD. Exchange experiments, with [¹⁴C]DCCD, [¹⁴C]DDT and unlabelled DDT at different concentrations, indicated that DDT and DCCD may be acting on a similar protein. This protein may act as the energy transducing protonophore required for the synthesis and hydrolysis of ATP in coupled mitochondria. Inhibition of mitochondrial ATPase activity may be a consequence of DDT and DCCD binding to this proteolipid protonophore, resulting in the disruption of energy transduction in muscle and nerve.     

Studies on the mode of action of asulam,aminotriazole and glyphosate in Equisetum arvense L. (field horsetail). I: The uptake and translocation of [14c]asulam, [14C]aminotriazole and [14C]glyphosate

The uptake and translocation of [¹⁴C]asulam (methyl 4-aminophenyl-sulphonylcarbamate), [¹⁴C]aminotriazole (1-H-1,2,4-triazol-3-ylamine) and [¹⁴C]glyphosate (N-(phosphonomethyl)glycine) were assessed in Equisetum arvense L. (field horsetail), a weed of mainly horticultural situations. Under controlled-environment conditions, 21°C day/18°C night and 70% r. h., the test herbicides were applied to 2-month-old and 2-year-old plants. Seven days following the application of 0.07-0.09 °Ci (1.14mg) of the test herbicides to young E. arvense, the accumulation of ¹⁴C-label (as percentage of applied radioactivity) in the treated shoots, untreated apical and basal shoots was as follows: [¹⁴C]asulam, 13.2, 0.18 and 1.02%; [¹⁴C] aminotriazole, 67.2, 3.65 and 1-91%; [¹⁴C]glyphosate, 35.9, 0.06 and 0.11%. The equivalent mean values for the accumulation of ¹⁴C-label in 2-year-old E. arvense were [¹⁴C]asulam, 12.0, 1-15 and 1.74%; [¹⁴C]aminotriazole, 58.6, 9.44 and 4.12%; [¹⁴C]glyphosate, 33.1, 0.79 and 2.32%. In the latter experiment, test plants received 0.25-0.30 °Ci (4mg) of herbicide, they were assessed after a 14-day period and the experiment was carried out at 3-week intervals between 2 June and 25 August on outdoor-grown plants. Irrespective of test herbicide or time of application, very low levels of ¹⁴C-label accumulated in the rhizome system. Only 0.2% of the applied radioactivity was recovered in 2-year-old plants and 0.4% in 2-month-old plants. In the young plants [¹⁴C]asulam accumulated greater amounts and concentrations of ¹⁴C-label in the rhizome apices and nodes than [¹⁴C]aminotriazole or [¹⁴C]glyphosate treatments. Inadequate control of E. arvense under field conditions may be due to limited basipetal translocation and accumulation of the test herbicides in the rhizome apices and nodes.     

Metabolism of [14C]dieldrin in bluegill fish

The exposure of bluegill fish to 50 parts per billion [¹⁴C]dieldrin in a static system resulted in the absorption of 73.00% of the radioactivity in 48 hr. Following transfer of the fish to clean water, only 16.20% of the absorbed radiolabel was eliminated in 23 days. Out of the 93.65% of the absorbed radioactivity recovered, 9 radioactive spots were isolated which included unchanged dieldrin (74.39%), pentachloroketone (8.17%), and aldrin-trans-diol (8.04%) as major metabolites.     

[14C]Glyphosate transport in undisturbed topsoil columns

Although glyphosate (N‐(phosphonomethyl)glycine) is one of the most frequently used herbicides, few controlled transport experiments in undisturbed soils have been carried out to date. The aim of this work was to study the influence of the sorption coefficient, soil‐glyphosate contact time, pH, phosphorus concentration and colloid‐facilitated transport on the transport of [¹⁴C]glyphosate in undisturbed top‐soil columns (20 cm height × 20 cm diameter) of a sandy loam soil and a sandy soil. Batch sorption experiments showed strong Freundlich‐type sorption to both soil materials. The mobility of glyphosate in the soil columns was strongly governed by macropore flow. Consequently, amounts of glyphosate leached from the macroporous sandy loam soil were 50–150 times larger than from the sandy soil. Leaching rates from the sandy soil were not affected by soil‐glyphosate contact time, whereas a contact time of 96 h strongly reduced the leaching rates from the sandy loam soil. The role of pH and phosphorus concentration in solution was relatively unimportant with respect to total glyphosate leaching. The contribution of colloid‐facilitated transport was <1 to 27% for the sandy loam and <1 to 52% for the sandy soil, depending on soil treatment. The risk for glyphosate leaching from the top‐soils seems to be limited to conditions where pronounced macropore flow occurs shortly after application. © 2000 Society of Chemical Industry     

Fate of [14C]diflubenzuron on cotton and in soil

Aqueous suspensions and oil emulsions of a commercial [¹⁴C]diflubenzuron (N-[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide) formulation (Dimilin W-25) remained on the leaf surface of greenhouse-treated plant tissues. Absorption, translocation, and metabolism of the [¹⁴C]diflubenzuron were not significant. Less than 0.05% of the applied ¹⁴C was found in newly developed plant tissues 28 days after spray treatment. [¹⁴C]Diflubenzuron was degraded in soil. After 91 days, biometer flask studies showed that 28% of the ¹⁴C incorporated into the soil as [¹⁴C]diflubenzuron was recovered as ¹⁴CO₂. Major dichloromethane-soluble soil residues were identified as unreacted [¹⁴C]diflubenzuron and [¹⁴C]4-chlorophenylurea. A minor unknown degradation product cochromatographed with 2,6-difluorobenzoic acid. Insoluble ¹⁴C-residues increased with time and represented 67.8% of the residual ¹⁴C in the soil 89 days after treatment. Cotton plants grown for 89 days in [¹⁴C]diflubenzuron-treated soil contained only 3% of the ¹⁴C applied to the soil. Small quantities of acetonitrile-soluble [¹⁴C]4-chlorophenylurea were isolated from the foliar tissues. Root tissues contained small amounts of [¹⁴C]diflubenzuron and trace quantities of a minor ¹⁴C-product that chromotographed similarly to 2,6-difluorobenzoic acid. Most of the ¹⁴C in the plant tissues (84–93%) was associated with an insoluble residue fraction 89 days after treatment.     

Metabolism of fungicidal cyanooximes, cymoxanil and analogues in various strains of Botrytis cinerea

BACKGROUND: The metabolism of cymoxanil [1‐(2‐cyano‐2‐methoxyiminoacetyl)‐3‐ethylurea] and fungicidal cyanooxime analogues was monitored on three phenotypes of Botrytis cinerea Pers. ex Fr. differing in their sensitivity towards cymoxanil. For this purpose, labelled [2‐¹⁴C]cymoxanil was added either to the culture medium of these strains or to its cell‐free extract. RESULTS: In the culture medium of the most sensitive strain, four main metabolites were detected. Three were isolated and identified. Cymoxanil was quickly metabolised by at least three concurrent enzymatic pathways: (i) cyclisation leading, after hydrolysis, to ethylparabanic acid, (ii) reduction giving demethoxylated cymoxanil, (iii) hydrolysis followed by reduction and then acetylation leading to N‐acetylcyanoglycine. In the cell‐free extract of the same strain, only the first and the second of these enzymatic reactions occurred. By comparing the metabolic profile of the most sensitive strain with that of the less sensitive ones, it was shown that the decrease in sensitivity to cymoxanil correlates with a reduced acetylcyanoglycine formation. Among all metabolites, only N‐acetylcyanoglycine is active against the most sensitive strain. Moreover, in a culture of this strain, two other fungicidal cyanooximes were also metabolised into this metabolite. CONCLUSION: The formation of N‐acetylcyanoglycine may play an important role in the fungitoxicity of cymoxanil and cyanooxime derivatives. Copyright © 2008 Society of Chemical Industry     

Movement and metabolic fate of [14C]ethephon in flue-cured tobacco

The absorption, distribution, and metabolic fate of [¹⁴C]ethephon in flue-cured tobacco (Nicotiana tabacum L.) was studied using autoradiography, thin-layer chromatography, high-voltage paper electrophoresis, and liquid scintillation spectrometry. Labeled ethephon penetrated mature leaf tissue easily and was translocated primarily in an acropetal direction. No ¹⁴C activity was detected in any other plant part except the treated leaf. The first day after treatment, most of the translocated ¹⁴C was detected in the midrib, and after 2 days radioactivity was noticed in veinal areas distal to the point of application. Four days later, however, ¹⁴C was detected in slight amounts only in the midrib, indicating that [¹⁴C]ethephon was rapidly degraded by the leaf tissue. Depending on leaf position on the stalk, as much as 92% of the radioactivity had disappeared from the leaf tissue during the first day after treatment, and as little as 5% of the applied radioactivity was recovered 4 days later. Methanol-extracted plant residues contained insignificant amounts of ¹⁴C. All of the ¹⁴C in methanol extracts was present in the form of a labeled compound with an R_f value corresponding to that of ethephon, indicating the absence of any detectable metabolites of the parent compound. Smoke analysis of cigarettes showed that more [¹⁴C]ethylene than ¹⁴CO₂ was recovered in the main stream, whereas the trend was reversed in the case of side stream smoke.     

Degradation of [14C]methoprene in the imported fire ant,Solenopsis invicta

The insect growth regulator, methoprene, is active against the imported fire ant, Solenopsis invicta Buren. This report presents data on the degradation of [¹⁴C]methoprene in the imported fire ant. The relative rates of absorption, metabolism, and excretion of methoprene in imported fire ant stages were found in the following order: adults>larvae>pupae>pharate pupae. Metabolism of methoprene by adults and pupae was primarily by O-demethylation to yield the alcohol-ester. Larvae and pharate pupae metabolized methoprene principally by esterase cleavage of the isopropyl ester which yielded the ether-acid. In larvae and pharate pupae there was a rapid increase in methoprene metabolism prior to the pharate pupal transformation and pupal molt. During the molt from pharate pupae to pupae, metabolism of methoprene appeared to occur in the cuticle, resulting in the accumulation of metabolites within and on the cuticle. The retention of these more polar compounds in the cuticle may contribute to the effectiveness of the insect growth regulator. The adults have a direct role in distributing the insect growth regulator and its metabolites, whereas the immatures may serve as a reservoir for maintenance of effective levels of insect growth regulator and active metabolites in the colony.     

The breakdown of [14C]monocrotophos insecticide on maize,cabbage and apple

The breakdown of ¹⁴C-labelled monocrotophos (dimethyl 3-hydroxy-N-methyl-cis-crotonamide phosphate) on the foliage of maize, cabbage and apple and on apple fruit has been studied. The breakdown products were mainly hydrophilic compounds such as ( II ) and probably dimethyl phosphate. Other breakdown products were ( III ), ( IV ) (free and conjugated), ( VI ) and ( IX ). In maize and apple ( IV ) was conjugated with sugars other than ß-D -glucose and a polar component, possibly a conjugate, was present in cabbage but this may not have been derived from ( IV ) .     

Effects and possible mode of action of some nonionic surfactants on the diffusion of [14C]glyphosate and [14C]Chlorotoluron acorss isoltated plant cuticles

The following are extended summaries based on posters presented at the 8th International Congress of Pesticide Chemistry (IUPAC), held in Washington, DC, USA, 4–9 July 1994. They are entirely the responsibility of the authors, and do not necessarily reflect the views of the Editorial Board of Pesticide Science.     

Uptake and distribution of [14C]metalaxyl by detached tobacco leaves

When the petioles of detached tobacco leaves (10–17 cm²) were incubated in aqueous solutions containing [¹⁴C]metalaxyl, uptake of the fungicide was dependent on the temperature and photoperiod. Detached leaves took up 78% more [¹⁴C]metalaxyl at 26°C than at 16°C. The rate of uptake in the light at 21°C was linear, but after an additional 20h in the dark, there was only twice as much fungicide in the leaves. Different sized leaves contained the same amount of fungicide per cm² area. Uptake by detached leaves of the ¹⁴C-labelled anilide lactones ofurace and RE-26940 [2-methoxy-N-(tetrahydro-2-oxo-3-thienyl)acet-2′,6′-xylidide] was similar to that of metalaxyl. At the concentration of metalaxyl (66 ng ml^?1) that controlled blue mould (Peronospora tabacina) on detached tobacco leaves, the amount of fungicide in the leaves was found to be 7.25 ng. Autoradiography showed that the distribution of [14C]metalaxyl in detached leaves after incubation for 23h was uniform, although higher concentrations of the label were present in the smaller veins of the leaves.