Oestrogen-priming of the hypophysectomized immature female rat promotes preantral follicle development in the absence of endogenous gonadotrophins and such as animal is useful for study of the intraovarian glycoprotein-steroidal hormone interactions which underlie morphological and functional development of the ovarian follicle. The present report identifies in vitro the functional characteristics (gonadotrophin binding and steroidogenesis) of granulosa cells harvested at different stages of follicular maturity following treatment with exogenous hormones in vivo. Preovulatory follicle maturation, induced by FSH, has been studied up until antrum formation and the acquisition by granulosa cells of the ability to respond directly to LH or hCG. Before the increases in available granulosa cell membrane LH/hCG receptors associated with the formation of follicular antra, effects of hCG or other hormones with interstitial cell stimulating activity are mediated via interactions with cells outside the lamina basalis. In-vivo studies with oestrogen-primed hypophysectomized immature rats indicate that androgens secreted by LH/hCG-stimulated thecal and/or interstitial cells may act directly on the preantral follicle to promote atresia. However, in-vitro studies have shown a stimulatory effect of androgen on FSH-responsive progesterone secretion by granulosa cells isolated from preantral follicles. These effects, if shown to operate within the ovary during the normal cycle, need not be mutually exclusive because FSH stimulation of granulosa cells in vivo may be a major determinant of follicular responses to androgen. The increase in follicle size and antrum formation accompanying FSH treatment in vivo are associated with (i) increases in the steroidogenic potential of isolated granulosa cells; (ii) the induction of granulosa cell LH/hCG receptors and of steroidogenic responsitivy to hCG; and (iii) stimulation of granulosa cell aromatase activity. These observations highlight the critical role of FSH in the organization of preovulatory follicular morphology and function.