Sustained EGF receptor (EGFR) phosphorylation by de novo synthesis of EGFR ligands plays an essential role in mediating luteinizing hormone (LH)-induced ovulation process in the preovulatory follicles (POFs). In the present study, the effect of epiregulin (EREG) on oocyte maturation and ovulation was investigated using Ereg knockout (Ereg(-/-)) mice congenic on a C57BL/6 background. Rate of spontaneous oocyte meiotic resumption of denuded oocytes (DOs) or cumulus cell-oocyte complexes (COCs) in vitro is similar between wild-type and Ereg(-/-) ice. However, gonadotropin-induced meiotic resumption in vivo is attenuated, and the number of COCs with expanded cumulus matrix and superovulated eggs dramatically decrease in Ereg(-/-) mice. Nonetheless, the number of eggs ovulated during normal estrus cycles and litter sizes in Ereg(-/-) mice are comparable to those of wild-type littermates. In contrast to other EGFR ligands, induction of amphiregulin (Areg) mRNA is severely reduced in ovaries collected from Ereg(-/-) mice either after human chorionic gonadotropin (hCG) treatment in immature mice or LH surge in adults. Gonadotropin-induced EGFR and ERBB2 phosphorylation in ovaries is attenuated in immature Ereg(-/-) mice, and MAPK3/1 phosphorylation and prostaglandin synthase 2 (PTGS2) protein levels are reduced. This attenuation, however, is no longer detectable in adult Ereg(-/-) mice after LH surge. This study implicates that EREG mediates signals downstream of Areg mRNA expression and that EGFR-ERBB2 signals contributes to regulation of ovulation process. (C) 2011 Elsevier Inc. All rights reserved.