Although equine chorionic gonadotropin (eCG) combined with gonadotro-pin-releasing hormone (GnRH) is known to improve reproductive rates by increas-ing the numbers of ovulating oocytes, the actions of these hormones on embryo im-plantation have not been elucidated. We herein eliminated the effects of the embryos by performing embryo transfer and elucidated the effects on embryo implantation after hormonal treatment. The recipient mice were allocated to four groups: a Con-trol group, an eCG group, a GnRH group, and an eCG+GnRH group. Well-developed blastocysts were transferred into recipients, and implantation sites were counted 5 days after embryo transfer. Our results revealed that embryo im-plantation rates in the eCG and eCG+GnRH groups were significantly lower than GnRH and Control groups (p < 0.01); eCG and eCG+GnRH groups didn’t differ (p > 0.05); and the implantation rate was lowest in the eCG+GnRH group. When we evaluated estrogen and progesterone concentrations in the four groups on E0.5 to E3.5, we noted that serum estrogen levels in eCG+GnRH group were signifi-cantly higher than in other groups (p < 0.01), while progesterone levels were sig-nificantly lower than in the Control group (p < 0.01), and that the progester-one/estrogen ratio was significantly reduced relative to the Control group (p < 0.01). Finally, the expression of the endometrial receptivity-related Lif gene was signifi-cantly diminished in the eCG+GnRH and eCG groups compared to other groups (p < 0.01). These results suggest that eCG combined with GnRH as a superovulatory regimen reduced mouse embryo implantation rates via an es-trogen-mediated im-pairment of endometrial receptivity.