Seasonal breeding in the ewe is regulated by photoperiod through a pineal-dependent mechanism. Changes in the ability of estradiol to inhibit tonic LH secretion are critical. During anestrus, this ovarian steroid gains the ability to slow the frequency of pulsatile LH secretion through an action on the brain. Exposure of ovariectomized, estradiol-implanted ewes to short photoperiods during summer anestrus revealed that daylength can control LH pulse frequency. After removal of estradiol, LH pulse frequency still differed between long- and short-day ewes, suggesting photoperiodic modulation of LH and presumably GnRH secretion independent of gonadal steroids. Significantly, the effects of daylength expressed both in the presence and the absence of estradiol failed to occur in pinealectomized ewes. Long-term infusions of melatonin, given in physiological patterns to pinealectomized ewes, mimicked the effects of photoperiod on pineal-intact ewes. Specifically, a pattern of melatonin characteristic of that in short days (16-hour night-time rise) led to an increase in LH pulse frequency to a breeding season rate. Conversely, melatonin infusions typifying a long-day pattern (8-hour night-time rise) produced an anestrous pulse pattern. Pituitary sensitivity to GnRH was not reduced in sheep which were reproductively suppressed by photoperiod or melatonin treatments. These observations support the conclusion that day-length acts through pineal melatonin secretion to regulate a neural LH pulse generator which, by changing the frequency of GnRH pulses, determines the ewe's seasonal reproductive state.