Testosterone was administered to female rats in high (1.25 mg) and low (10 µg) doses at different stages of development to determine if the well-known changes in reproductive function following such treatment are accompanied by altered serotonin metabolism in the pineal gland. Testosterone (1.25 mg) given on day 3 of life depressed pineal serotonin levels and abolished pineal serotonin circadian rhythm at 7, 20 and 60 days of age. A comparable dose of testosterone given on day 13 depressed the serotonin rhythm at day 20, but the rhythm was restored at day 60. 60-day-old rats which received testosterone (1.25 mg) on day 13 showed regular estrous cycles, while the neonatally treated rats were acyclic. Therefore, the sterilizing effect of testosterone also prevented development of daily pineal serotonin rhythms