In the monkey, estradiol appears to exert both its negative and positive feedback actions on gonadotropin secretion primarly at the hypophysial level. In the rat, inhibitory and stimulatory effects of estradiol on the pituitary are also evident, but additional actions on the medial basal hypothalamus (MBH) and preoptic area must be postulated to fully account for the negative and positive feedback effects of estradiol in this species. The available evidence points to the MBH as a site for the negative feedback action of progesterone in the rat. In both species, progesterone probably acts in the central nervous system to block the positive feedback action of estradiol while the facilitation of luteinizing hormone release by this steroid appears to be at a hypophyisial level.