The effects of insulin-like growth factors (IGF-I and II) were compared with those of insulin in porcine granulosa cells in vitro with regard to stimulation of thymidine incorporation, facilitation of FSH-induced progesterone secretion, and displacement of [125I] IGF-I from specific receptors. In all three assays, the order of potency was IGF-I from specific receptors. In all three assays, the order of potency was IGF-I greater than IGF-II greater than insulin. However, the difference in potency was marginally significant in the mitogenic assay. These data are consistent with the presence of specific IGF-I receptors on these cells and involvement of these binding sites in regulation of granulosa cell replication and differentiation.