Effect of melatonin administered during the night on reproductive function in the Syrian hamster.

Syrian hamsters were maintained on a long photoperiod (lights on 0600-2000 h) and injected daily with melatonin (10 microgram per injection) for 6-7 weeks. Females injected at 2000 h or 0400 h ceased estrous cyclicity, and males treated similarly at these times had reduced testis weights by the sixth week of treatment. Daily administration of the same dosage of melatonin at 2400 and 0100 h failed to interrupt estrous cyclicity or reduce testis weight. These observations suggest that exogenous melatonin administered at 2000 h or 0400 h may be temporally adding to endogenous circulating melatonin, inducing gonadal quiescence.