Melatonin, a pineal substance: effect on the rat ovary

Daily injection of microgram amounts of melatonin in rats decreased the incidence of estrus and reduced ovarian weight. Circulating melatonin was selectively taken up and retained by the ovary and pineal gland; this effect was reduced by exposure of rats to constant light. A single injection of melatonin lowered the incidence of estrus among rats exposed to constant light.