Mice administered an inhibin antiserum, equine chorionic gonadotropin (eCG) mixture called CARD HyperOva (OVA)/human chorionic gonadotropin (hCG), ovulate more oocytes than those administered eCG/hCG. In this study, the mechanism by which more oocytes are ovulated was investigated. Apoptotic cells were not observed in ovaries 24 and 48 h after OVA injection, and INHBA expression was absent in the growing follicles with apoptotic cells, whereas granulosa cells in follicles expressing INHBA also expressed estrogen receptor α (ESR1). ESR1 and CYP19A1 expression and 17β-estradiol (E