In this presentation experimental evidence is reviewed and new information is presented concerning the question of cyclic fluctuations in pituitary responsiveness to endogenous and exogenous hypothalamic-releasing hormones as well as the direct role of steroids in modulating pituitary sensitivity. Experiments are described which show that four-day and five-day cyclic rats respond differently to LH-RH, presumably because of differences in secretion patterns of ovarian steroids. The direct role of these steroids in this process was investigated both from the morphological (light and electron microscopy) and physiological (radioimmunoassay measurement of LH) viewpoint in hypophysectomized rats bearing pituitary homografts. It was found that steroids may directly alter both the morphology and the secretory function of pituitary gonadotrophs. Estradiol was found to depress the amount of LH that was secreted while progesterone was seen to influence primarily the time of LH release following LH-RH injection. Thus, it appears that direct feedback effects of ovarian steroids at the level of the pituitary are important regulators of the sensitivity of pituitary gonadotrophs to the LH- releaseing action of hypothalamic LH-RH.