BACKGROUND Endometrial development has an important role in blastocyst adhesion and implantation. During IVF cycles, endometrial development is enhanced by progesterone. OBJECTIVE The aim of this study was to compare ultrastructural and morphometrical characteristics of mice uterine endometrium in natural cycle with those in superovulated cycles received progesterone or Sildenafil. METHODS In This study, 60 female bulb/c mice were divided into 4 groups: a control and 3 experimental; gonadotropin, gonadotropin+ Sildenafil and gonadotropin+ progesterone. In experimental groups the mice superovulated mated. In the gonadotropin+ progesterone and gonadotropin+ Viagra groups, the mice respectively received 1mg progesterone and 3 mg Sildenafil citrate. Their uterine specimens were prepared for morphometrical and ultrastructural study. Height of the epithelial cells was measured, using motic software. Statistical analysis was performed using ANOVA. RESULTS Microscopy revealed that in control group the cells had numerous apical microvilli and the height of the cells was 20.52±2.43 µm. In gonadotropin+ progesterone group, the granules were found in basal and apical portions and cellular height were 17.91±2.78 µm which were significantly shorter than in the control and gonadotropin groups (p<0.001). In this group, the apical membrane also contained pinopodes. In gonadotropin +Sildenafil group, the granules were found in both apical and basal portions and the height of the cells were 17.60±2.49 µm which were significantly shorter than in the control and gonadotropin groups (p<0.001). In this group, pinopodes appeared slightly extensive than the other groups. CONCLUSIONS It is concluded that superovulatory drugs in mice stimulate endometrial maturation but injection of Sildenafil is nearly more positive. This article extracted from Ph.D. thesis. (Bahman Rashedee).