Recent evidence has been presented that follicle-stimulating hormone (FSH) stimulates the induction of granulosa cell c-fos protooncogene mRNA in vivo (Pennybacker and Herman (1989) J. Cell Biol. 109, 151A; Delidow et al. (1990) Endocrinology 126, 2302-2306), yet the mechanisms by which FSH induces c-fos mRNA expression have not been delineated. To elucidate the mechanisms of FSH-dependent c-fos mRNA expression, we measured the time and dose dependence of c-fos mRNA levels using Northern blot analysis in intact ovaries and cultured granulosa cells in response to FSH. In intact ovaries, FSH-induced c-fos mRNA expression was time dependent with maximal expression at 90 min post FSH injection, while in cultures of granulosa cells obtained from estrogen-primed immature female rats, c-fos mRNA levels were highest after 30 min exposure to FSH and at a concentration of 100 ng/ml. Neither 8-bromo adenosine 3',5'-cyclic monophosphate (8-br-cAMP), at doses ranging from 0.1 to 10 mM, nor 100 microM forskolin (in the presence or absence of 200 microM isobutyl-methylxanthine) or luteinizing hormone (LH, 100 ng/ml) were able to mimic FSH-induced c-fos mRNA expression in granulosa cell cultures. However, tetradecanoyl-13-phorbol acetate (TPA, 200 nM) was able to induce c-fos mRNA expression. The protein kinase C (PKC) inhibitors H-7 (0.3-30 microM) and staurosporine (0.75 micrograms/ml) blocked FSH-induced c-fos mRNA expression in cultured granulosa cells while HA 1004, an inhibitor of cGMP- and cAMP-dependent protein kinases at 30 microM had no effect on TPA-induced c-fos expression, and only minimally inhibited FSH-induced c-fos expression. Both FSH (100 ng/ml) and forskolin (3 microM) increased progesterone production in cultured granulosa cells. These data support the hypothesis that FSH specifically induces c-fos mRNA expression by a PKC-dependent mechanism and that the cAMP arm of the FSH response pathway is operant in these cells.