Epidermal growth factor (EGF) modestly increased DNA synthesis by cultured rat granulosa cells. FSH also stimulated DNA synthesis dose-relatedly with the maximal effect occurring at FSH 100 ng/ml. The stimulatory effect of FSH was still greater than that of EGF. However, in the presence of EGF, the stimulatory effect of FSH at any concentration was regulated to the level as high as when EGF alone stimulates. In addition, EGF inhibited DNA synthesis induced by forskolin, but enhanced the action of (Bu)2cAMP additively, which indicates that EGF attenuates DNA synthesis of granulosa cells by suppressing the activity of adenylate cyclase or cAMP production. As it is suggested that cAMP is the most likely intracellular second messenger for FSH, the regulatory effect of EGF on FSH-induced DNA synthesis could be, in part, due to suppression of cAMP production. These results suggest that EGF is involved in granulosa cell proliferation, irrespective of the presence of FSH, in a different pathway from FSH and additionally modulates the FSH growth-promoting effect as a local regulator. The interaction between EGF and FSH may be important in the control of follicular development.