The role of estradiol and progesterone in the feedback control of FSH secretion during the ovine estrous cycle was examined in three experiments. In the first, FSH was monitored in serum obtained every 4 h throughout the normal cycle. Although there was considerable variability among animals, the mean FSH concentration was elevated 1-2 days after estrus, then declined gradually to a midluteal phase nadir, and remained there until the preovulatory FSH surge on estrus. In the second study, the contribution of estradiol and progesterone to the determination of this pattern was analyzed. Ewes were ovariectomized 2 days after estrus, and the physiological pattern and level of each steroid were reproduced, alone or in combination, by sequential additions and removals of Silastic implants. None of the treatments maintained physiological concentrations of FSH, although some suppression by steroids was evident. In contrast, the time course of LH in ewes receiving estradiol plus progesterone was virtually identical to that in intact ewes. In the last study, the feedback actions of the preovulatory estradiol rise were examined by producing a variety of physiological estradiol increments under conditions which simulated the follicular phase of the cycle. Again, the estradiol rise could not account for the low basal level of FSH typical of the follicular phase, whereas basal LH was normal. A normal FSH surge, however, was reliably induced by estradiol. These results support the hypothesis that an ovarian hormone other than estradiol or progesterone contributes to the feedback control of tonic FSH secretion during the ovine estrous cycle. Further, since these two steroids can account for secretion of LH, this putative hormone most likely selectively suppress FSH.