Mutations in the bone morphogenetic protein 15 (BMP-15) gene cause female infertility in the monoovulatory human and sheep; however, in the polyovulatory mouse, loss-of-function of BMP-15 results only in reduced ovulation rate. To elucidate the cause of these species-specific differences, we investigated the functional role of BMP-15 in the mouse ovary. Here, we found that the functional mature form of BMP-15 is barely detectable in the mouse oocytes until just before ovulation, when it is markedly increased. Further, we found that BMP-15 induces cumulus expansion in mouse cumulus-oocyte complexes. The oocyte culture medium from immature mice primed with pregnant mare serum gonadotropin followed by human chorionic gonadotropin also stimulated cumulus expansion, and this activity was attenuated by BMP-15 antibody. Interestingly, the oocyte culture medium from mice treated with pregnant mare serum gonadotropin alone had no effect. Moreover, BMP-15 stimulated the expression of EGF-like growth factors in cumulus cells as well as a series of molecules downstream of EGF-like growth factor signaling, including cyclooxygenase 2, hyaluronan synthase 2, tumor necrosis factor-stimulated gene 6, and pentraxin 3, all of which are necessary for normal cumulus expansion. An antagonist of the EGF receptor completely abolished the effect of BMP-15 in inducing cumulus expansion. These results are consistent with the phenotype of BMP-15-null mice, which exhibit normal folliculogenesis but have defects in the ovulation process. The species-specific differences in the phenotypes caused by BMP-15 mutations may thus be attributed to the temporal variations in the production of the mature form of BMP-15.