The purpose of this study was to determine if the temporal relationship between the LH surge and the photo-period is influenced by melatonin and the pineal gland in the rat. The timing of melatonin synthesis and of preovulatory LH release is reciprocally related; and since melatonin blocks LH secretion, it was hypothesized that the pineal ‘hormone’ contributes normally to termination of the LH surge. To test this hypothesis, we examined the temporal pattern of phasic LH secretion after altering pineal activity in rats with regular estrous cycles. Termination of the LH surge was delayed in rats exposed to a photophase acutely prolonged into the night of proestrus; a treatment which delays melatonin synthesis. On the other hand, the LH surge terminated normally despite extended light exposure, if the rats were injected with melatonin at 19.00 h, when the dark phase usually begins. Melatonin injections during the standard photophase, i.e. at 13.00 or 17.00 h, blocked or prematurely terminated the LH surge, respectively. However, these effects probably do not represent a physiological function of melatonin, since its synthesis does not accelerate normally until after dark. 5 out of 14 pinealectomized rats showed altered patterns of preovulatory LH secretion 2 months after surgery. Since melatonin has been reported to block serotonin receptors, and serotonin stimulates phasic secretion of LH, we tested the ability of quipazine, a serotonin receptor agonist, to antagonize the inhibitory effect of melatonin on the LH surge. Quipazine given at 15.30 h blocked the inhibitory effect of melatonin administered at 16.00 h. The serotonin agonist partially restored LH levels in proestrous rats when it was given 30 min after melatonin. Therefore, the inhibitory effect of melatonin on phasic secretion of LH may result from an action upon serotonin-containing neurons. These findings taken together suggest that the pineal gland, ostensibly through the action of melatonin, contributes to termination of phasic LH secretion and thereby participates in entrainment of the LH surge to the photoperiod in the female rat.