The effects of mifepristone (RU486) on the early and middle stage of pregnant rats were examined using light and electron microscopy. Two groups of pregnant rats were administered with RU486, at a dose of 1. 25 mg/rat on days 7 and 10 of pregnancy, respectively. Thereafter all the rats with middle stage had abortion with necroticdecidua at 48h post administration. However, uterus endometrium returns to normal. The number of the lipid droplets in the luteal cells increased. The level of serum progesterone was significantly lower than that of the control group. For rats with early stage pregnancy, the development of some embryos in two cases was similar to that of the control group at 48h post administration. The embryos of the remaining rats in the group were all disintegrated. Nevertheless, two rats had their uteri restored. There were no significant differences in the structure of corpus luteum, the levels of hypophyseal gonadotrophin nor the serum progesterone between the treated and the control groups. These results indicate that the anti-pregnant activity of RU486 is more effective during the middle stage pregnancy than in the early pregnancy in rats. The effects of mifepristone (RU486) on the early and middle stage of pregnant rats were examined using light and electron microscopy. Two groups of pregnant rats were administered with RU 486, at a dose of 1. 25 mg / rat on days 7 and 10 of However, uterus endometrium returns to normal. The number of the lipid droplets in the luteal cells increased. The level of serum progesterone was significantly lower than that that For the control group. For rats with early stage pregnancy, the development of some embryos in two cases was similar to that of the control group at 48h post administration. The embryos of the rats in the group were all disintegrated. Nevertheless, two rats had their uteri restored. There were no significant differences in the structure of corpus luteum, the levels of hypophyseal gonadotrophin nor the serum progesterone between the treated and the control groups. These results indicate that the anti-pregnant activity of RU486 is more effective during the middle stage pregnancy than in the early pregnancy in rats.