Granulosa cells were isolated from ovaries of estrogen-treated immature intact and hypophysectomized rats, and were maintained in culture in a chemically-defined medium. Progesterone secretion by these cells was stimulated by the addition of FSH [follicle stimulating hormone] to the medium. In the presence of 0.5 .mu.M testosterone or 17.beta.-OH-5.alpha.-androstan-3-one (DHT), progesterone secretion was low or undetectable. The addition of testosterone or DHT together with FSH caused a dramatic 8- to 19-fold increase over that caused by FSH alone. Luteinizing hormone alone had no effect on progesterone secretion, but produced a small stimulation when added together with testosterone. These results demonstrate synergism between androgens and FSH in the control of progesterone secretion by granulosa cells in culture.