Roles of the p38-MAPK pathway in steroidogenesis were investigated using coculture of rat granulosa cells with oocytes. Activin and FSH readily phosphorylated p38 in granulosa cells. Activin effect on p38 phosphorylation was abolished by a selective activin receptor-like kinase-4, -5, and -7 inhibitor, SB431542. SB431542 decreased FSH-induced estradiol but had no effect on progesterone production with a marginal cAMP reduction, suggesting that endogenous activin is primarily involved in estradiol synthesis. FSH-induced p38 activation was not affected either by SB431542 or follistatin, suggesting that FSH activates p38 not through the endogenous activin. Bone morphogenetic protein (BMP)-2 and BMP-4 also enhanced FSH-induced p38 phosphorylation, which was augmented by oocyte action. A specific p38 inhibitor, SB203580, decreased FSH-induced estradiol production. However, FSH-induced cAMP accumulation was not changed by SB203580, suggesting that p38 activation is linked to estradiol synthesis independently of cAMP. BMP-2 and BMP-4 inhibited FSH- and forskolin (FSK)-induced progesterone and cAMP synthesis regardless of oocyte action. BMP-2, BMP-4, and activin increased FSH- induced estradiol production, which was enhanced in the presence of oocytes. In contrast to activin that enhanced FSK-induced estradiol, BMP-2 and BMP-4 had no effects on FSK-induced estradiol production, suggesting that BMP-2 and BMP-4 directly activate FSH- receptor signaling. Given that activin increased, but BMP-2 and BMP-4 decreased, FSH- induced cAMP, the effects of BMP-2 and BMP-4 on estradiol enhancement appeared to be diverged from the cAMP-protein kinase A pathway. Thus, BMP-2 and BMP-4 differentially regulate steroidogenesis by stimulating FSH- induced p38 and suppressing cAMP. The former is involved in estradiol production and enhanced by oocyte action, whereas the latter leads to reduction of progesterone synthesis. (Endocrinology 150: 1921-1930, 2009)