Various growth factors are likely to be involved in the proliferation and differentiation of granulosa cells. Our previous studies revealed that insulin, insulin-like growth factor (IGF-I) and thyroid hormone each act as a biological amplifier of the action of FSH in the ovary. This study was undertaken to elucidate a possible direct effect of epidermal growth factor (EGF) on granulosa cell functions using the procine model. Porcine granulosa cells obtained from small, medium and large follicle, were cultured in the presence or absence of FSH and IGF-I, with or without EGF. EGF stimulated the proliferation of granulosa cells from small follicles, but did not affect the proliferation of granulosa cells from large follicles. In culture of granulosa cells obtained from large follicles. EGF inhibited the secretion of estradiol, and increased the secretion of progesterone by cultured cells. Scatchard analysis of 125I-EGF binding revealed that there is a single type of EGF receptor (EGF-R) in granulosa cells and that the number of EGF-R increases as the follicle matures. In Sephadex G-75 column chromatography of the medium following 5-day culture of porcine granulosa cells, an elution peak of immunoreactive EGF was found in the fraction similar to the elution of 125I-hEGF. These results suggested that EGF is produced by granulosa cells which contain abundant EGF-R. Thus, EGF is likely to act in an autocrine manner to stimulate the proliferation of less mature granulosa cells and to increase progesterone production by mature granulosa cells in association with an inhibitory action on estradiol production. These dual actions of EGF on granulosa cell function are of great interest.