This study determined whether melatonin's ability to block phasic secretion of LH is a positive timing factor in the reproductive cycle of the female rat. When melatonin synthesis was delayed by extending the photophase on proestrus, serum LH levels were significantly elevated at 2200h. However, melatonin given at 1900h depressed these levels to control values by 2200h despite the extended photophase. This suggests that melatonin helps terminate the LH surge. Since the effect was blocked by pretreatment with quipazine, a serotonin receptor agonist, melatonin may help terminate the LH surge by blocking serotonin neurotransmission. Furthermore, the effects of melatonin and methiothepin, a serotonin receptor antagonist, on hypothalamic serotonin metabolism were comparable, and opposite from quipazine. Temporal patterns of the LH surge changed two months after pinealectomy. These findings suggest that melatonin contributes to termination of the LH surge by an antiserotonin mechanism. The rising levels of melatonin after dark help terminate serotonin signals, and provide a definitive end point for the LH surge, thus helping entrain the gonadotropin rhythm to the light:dark cycle.