The melatonin content measured by radioimmunoassay of the pineal gland over a 24-hour period (a light:dark cycle of 14 hours of light and 10 of darkness) was compared in young and old female and male Syrian hamsters. The young animals of both sexes exhibited roughly an eightfold rise in pineal melatonin during the dark phase of the cycle, whereas in the old hamsters the nocturnal rise in melatonin was almost completely absent. The results indicate a marked drop in pineal biosynthetic activity in the aging hamster.