Summary. An immuno%gical study of melatonin in the pineal organ, retina and plasma of the pigeon.Circadian variations in melatonin content have been studied in the pineal organ, retina and plasma of adult pigeons using radioimmunoassay. A clear-cut peak of melatonin was observed in the pineal organ at middark ; this middark peak was of larger amplitude in July than in November. A nocturnal rise in melatonin concentration was also observed in the retina and plasma.We also used an immunocytochemical technique revealing the positive reactions of a melatonin-like compound in the photoreceptor cell layer (inner segments) of the pigeon's retina. Up to now, no melatonin storage has been visualized in the pineal organ of the pigeon.Introduction.