To examine the relationships between gonadotropic hormones, ovarian follicular development, and estradiol receptor content in rat granulosa cells, follicular growth was stimulated in intact immature rats and hypophysectomized immature rats. In intact rats, both PMSG and oFSH given for 2 days stimulated follicular growth and endogenous estrogen production but caused estradiol receptor content in granulosa cells to decrease markedly. In hypophysectomized rats estradiol (2 mg/day X 4) was capable of increasing the content of its own receptor, stimulated preantral follicular development, and potentiated the responses of granulosa cells to highly purified hFSH. Thus, subsequent treatment with hFSH stimulated antral formation and maintained high levels of estradiol receptor content in granulosa cells. LH, on the other hand, caused a rapid decline in the content of estradiol receptor in nuclei of granulosa cells. In hypophysectomized rats primed with estradiol alone, a high dose of oLH caused follicles to undergo atresia. In hypophysectomized rats primed with hFSH in addition to estradiol, oLH stimulated luteinization. Thus, induction of atresia or luteinization by oLH in these rats appears to be associated (cause or effect?) with a loss of estradiol receptor in granulosa cells. Whether follicles of estrogen-treated, hypophysectomized rats luteinize or undergo atresia in response to LH appears to be determined by their stage of differentiation and the nature of the prior stimulation by FSH.