In order to study the effect of progesterone in modulating the secretion of gonadotropins, various doses of progesterone were administered to castrated immature and mature female rats. Progesterone did not prevent the postcastration rise of FSH and LH in these animals. Various amounts of progesterone were then administered to castrated rats that were treated with a constant low dose of estradiol. In animals given doses of estradiol that were able to decrease serum FSH and LH below castrate levels, but not to prevent the post-castration rise, the effect of progesterone was dose-dependent. A very low dose of progesterone lowered serum gonadotropins, an intermediate dose brought about an increase in secretion, and high doses were suppressive. The modifying action of progesterone appears to be dependent upon the level of estrogen, the dose of progesterone, and the time of administration.