The purpose of this study was to determine the effects of Nilestriol on reproductive system. Female SD rats, aged 4 months, were randomly divided into three groups. The OVX group's rats were subjected to bilateral ovariectomy, the SHAM group's rats received sham surgery; the Nilestriol(CEE3) group's rats were subjected to bilateral ovariectomy and treated with CEE3(0.15 mg/100 g BW, once a week). A computerized image analyzer was used to evaluate the endometrial pathological changes 10 weeks later. The weight of uterus was measured. The results showed that in OVX group, atrophic uteri; and atrophied endometria were noted. The weight of uterus was increased in CEE3 group, compared with that in OVX group (642.20 +/- 283.60 mg vs. 297.00 +/- 106.90 mg, P < 0.05). The volume fraction of endometrial glands was also significantly elevated in CEE3 group, compared with that in OVX group (1.84% vs 0.58%). The endometria in CEE3 group were at an early proliferative stage. These suggest that Nilestriol has a slight stimulating effect on the uteri of ovariectomized rats, including their endometria and uterine smooth muscles. Nilestriol is suited to be used in combination with progestogen.