The potential role of the hypoxanthine-xanthine oxidase system in human chorionic gonadotropin-induced ovulation, nuclear maturation, and preimplantation development was investigated in the rabbit by use of the xanthine oxidase inhibitor allopurinol. Allopurinol (50 mg/kg) or vehicle was injected, followed by human chorionic gonadotropin (100 IU). Administration of inhibitor or vehicle was repeated 3 hours later. Ovaries and ovulated ova were inspected 12 or 24 hours after human chorionic gonadotropin administration for follicle rupture, cumulus dispersal, and nuclear maturation. Ova were inseminated in vitro and assessed for development to the blastocyst stage, up to 96 hours after insemination. Allopurinol significantly reduced ovulatory efficiency, defined as the percent of large follicles that ovulate, and decreased the percent of ova with cumulus dispersal. Allopurinol significantly inhibited morula and blastocyst formation. These data suggest that the xanthine oxidase system may play a role in ovulation and early embryonic development.