Endometrium of GnRH treated group resembled with pregnant group and endometrial thickness in these groups significantly increased in comparison with non-pregnant group. In GnRH treated animals, most of histomorphological changes in epithelial cells, glands and stroma of uterus was similar to pregnant group. The results revealed that mammalian form of GnRH exerted endometrial change in guinea pig almost similar to those occur in normal pregnant animals and its administration prior to implantation may improve pregnancy rate following embryo transfer.