Aim: This study was designed to investigate the effects of gonadotropin-releasing hormone agonist (GnRHa) on the ovulation rate, the oocyte number and maturation in mice superovulated with equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCG). Methods: Thirty female mice, aging 3 months and weighing between 25-30 g, were assigned to the three experimental groups of control, superovulated and GnRHa treated, each containing 10 animals. Control mice were received only the intraperitoneal (i.p.) injection of 0.1 ml 0.9% NaCl. Superovulated mice were given an i.p. injection of 5 IU of eCG followed 48 hr later by 5 IU of hCG injection (i.p.). The GnRHa treated mice were received an intramuscular (i.m.) injection of a 20 mg/kg GnRHa (buserelin) 24 hr before the superovulation with eCG -hCG. Thirteen hr after hCG administration, mice were sacrificed by cervical dislocation. The ovaries and the fallopian tubes were also harvested for counting the corpora lutea and cumulus-enclosed oocytes. Results: The oocyte number and the maturation, ovulation rate, and the corpus luteum number were higher in GnRHa treated mice than controls and superovulated mice. Conclusion: It is concluded that an i.m. injection of the GnRHa 24 hr before the superovulation with eCG-hCG increases the oocyte number and maturation, and ovulation rate, thus, could be used for in vivo and in vitro fertilizations.