The effect of constant light from birth on the appearance of a diurnal rhythm in serotonin content in the rat epiphysis.

The appearance of a diurnal rhythm in serotonin content in the rat epiphysis is suppressed by rearing the young in constant light from birth. If the young are put from constant light into constant darkness at age 20 days, by age 30 days there is a marked rhythm in the content of pineal serotonin. It would appear that this rhythm is inborn, since it appears even after an initial suppression without the intervention of an external synchroniser. Fifty-eight-day-old males, maintained in constant light from 21 to 11 days before killing, and then in constant darkness, showed a normal diurnal rhythm in epiphysial serotonin content.