This study was conducted to determine whether the pineal indoleamine melatonin mediates the effects of photoperiod on the capacity of estradiol to inhibit LH secretion in the ewe. Patterns of serum melatonin were characterized in pineal-intact ovariectomized ewes treated with sc Silastic estradiol implants and exposed to 90-day alternations between long and short photoperiods. High fluctuating levels of serum melatonin were found during the night, with the duration of elevated serum levels corresponding to the length of the dark period. Transfer from long to short photoperiods caused a rapid change in the duration of nightly melatonin secretion and reduced the negative feedback potency of estradiol upon LH secretion during the natural anestrous season. In pinealectomized ewes, the night-time rise of melatonin was absent, and transfer from long to short days failed to reverse the capacity of estradiol to inhibit LH secretion during anestrus. Nightly infusions of melatonin restored patterns of this indoleamine similar to those observed in pineal-intact ewes exposed to the 90-day alternation between long and short days. The melatonin infusions also restored the reproductive response to the inductive photoperiod: in every ewe, the negative feedback effects of estradiol upon LH secretion were diminished after transfer from long to short days. The amplitude and latency of this escape matched those of pineal-intact animals. We conclude that the pineal mediates the reproductive response of the ewe to inductive photoperiods through its daily rhythm of melatonin secretion.