Studies in vitro and in vivo indicate that steroid secretion by granulosa cells of the developing follicle is modulated in part by steroid hormones themselves. Androgens stimulate progestin synthesis in vitro by granulosa cells of all developmental stages independently of their role as an estrogen precursor; generally, the action of estradiol results in suppression of progesterone secretion. The nature of the relationship between steroid secretion by granulosa cells and the physiological processes of follicular development or atresia remains to be investigated in greater detail. The cellular mechanism(s) of steroidal modulation of steroid secretion is unknown but apparently does not involve changes in the number of granulosa cell LH receptors per se.