Porcine granulosa cells from small (1-2 mm), medium (3-5 mm), or large (6-12 mm) follicles were exposed to highly purified human FSH [follicle stimjlating hormone] LER 1801-3) and 17.beta.-estradiol in tissue cultrue. The cells were grown in TC 199 plus 4% pig serum, and progesterone secreted into the culture medium was measured by RIZ [radioimmunoassay]. Granulosa cells obtained from follicles at all 3 stages of development were stimulated by FSH to secrete progesterone. The stimulation was dependent on the concentration of FSH and the stage of follicular development. In the absence of any added hormones, the granulosa cells from small, medium, or large follicles in 6 experiments secreted (between days 2-4) 1.5 .+-. 0.1, 60 .+-. 3, and 152 .+-. 5 pg of progesterone/100 cells, respectively. Addition of 0.1 .mu.g/ml FSH led to a 553% 292% and 171% stimulation of progesterone secretion by cells from small, medium and large follicles, respectively. The granulosa cells from small follicles secreted progesterone at a steady rate throughout the 12-day culture period. FSH stimulation of progesterone secretion required the continued presence of the gonadotropin; removal of FSH resulted in a gradual decline in progesterone secretion which reached control levels within 6 days after removal of FSH. 17.beta.-Estradiol (1 .mu.g/ml) inhibited the FSH stimulation of progesterone accumulation by small follicle granulosa cells from 7.5 .+-. 0.3 to 3 .+-. 0.1 pg/100 cells. The withdrawal of estradiol on day 4 allowed FSH to stimulate progesterone secretion to normal levels by day 8. Testosterone mimicked 17.beta.-estradiol action, whereas dihydrotestosterone, cortisol, or 17.alpha.-estradiol were without detectable effect. Testosterone probably acts after being aromatized to estrogen. Intrafollicular estrogen may, along with other follicular fluid components, be responsible for keeping the granulosa cells from secreting progesterone in response to FSH during early follicular development. The mechanism of inhibitory action of estrogen upon progesterone accumulation in not known; it is possible that it may alter progesterone metabolism as well as its secretion. This possibility is currently being tested.