The pineal gland conveys photoperiodic information to the brain through its daily pattern of melatonin (MEL) secretion. The duration of MEL secretion is proportional to the duration of the night. To determine the mechanism by which MEL transduces photoperiod, we used a protocol of daily MEL infusion given to sexually active pinealectomized Syrian hamsters. A long MEL signal (10 h) inhibited sexual activity, whereas a 5-hour infusion had no effect. However, animals given a 2.5-hour infusion twice separated by an interval of 3 h produced complete gonadal atrophy. Changes in the time interval between infusions blocked the potency of the MEL infusion, suggesting a tight temporal relationship between MEL signals. Additionally, the infusions were as effective whether applied during the day or during the night, in both long and short photoperiods. These data suggest that there is a rhythm of sensitivity to MEL involved in the photoperiodic response which is entrained by MEL itself.