FSH promotes the differentiation of ovarian follicular granulosa cells via a cAMP-dependent mechanism. Based upon the presence of a prominent phospholipid/diolein/Ca2+-independent myelin basic protein kinase activity in soluble extracts of proliferating immature rat granulosa cells, we determine whether this activity was attributable to the mitogen-activated protein kinases (MAPKs), one of the ubiquitous families of myelin basic protein kinases, and whether FSH acutely regulated the MAPKs in rat granulosa cells. Granulosa cells were obtained from large preantral follicles in ovaries of immature rats treated with 17beta-estradiol to promote granulosa cell proliferation. Exposure of granulosa cells, cultured overnight in serum-free medium containing 10 nM 17beta- estradiol, to 50 ng/ml FSH for 10 min promoted a 2- to 5- fold increase in MAPK activity. The effects of FSH were mimicked by forskolin and inhibited by the inhibitor of cAMP- dependent protein kinase H89, but were not inhibited by the tyrosine kinase inhibitor Ag-18. FSH also promoted increased phosphorylation of the 90-kDa ribosomal S6 protein kinase and phosphorylation of exogenous S6 protein. These results suggest that the cAMP-directed pathway by which FSH initiates granulosa cell differentiation includes activation of MAPKs.