The purposes of the present study were to evaluate the effect of estrone on the prevention of implantation and pregnancy and the concentrations of estradiol and progesterone or sodium and potassium in the plasma or uterine fluid of pregnant rats injected a single dose of 0, 0.1, 0.4 or 1.0 mg on days 5, 9, or 13 of pregnancy. The body weight gains of pregnant rats were significantly decreased in administering estrone on day 5 of pregnancyi no effect obtained on days 9 or 13. Numbers of viable fetuses were significantly decreased and pre- and post-implantation loss increased by administering estrone 0.1, 0.4 or 1.0 mg and numbers of implantation sites significantly increased by administering estrone 0.1 mg but significantly decreased or not occurred by 0.4 or 1.0 mg on day 5 of pregnancy, respectively, but no effect obtained on days 9 or 13. Plasma estradiol concentrations of pregnant rats were significantly increased on 1, 2 or 18 days after treatment with estrone (1.4 or 1.0 mg and 3 days after 0.4 mg on day 5 of pregnancy, respectively. But plasma progesterone concentrations had no effect on the treatment with estrone. Plasma sodium contents were significantly increased on 1 day after treatment with estrone 0.1 or 1.0 mg and 3 days after 0.1 mg on day 5 of pregnancy, respectively, except potassium content were no detectable change. Sodium contents in uterine fluid were significantly increased on 1 day after treatment with estrone 1.0 mg and 3 days after 0.1, 0.4 or 1.0 mg on day 5 of pregnancy, respectively. And potassium contents were significantly increased on 1 or 2 days after treatments with estrone 0.1 mg but significantly decreased on 3 days after 0.4 or 1.0 mg on day 5 of pregnancy, respectively. It is suggested that the prevention of implantation and failure of pregnancy could be induced by treatment with estrone 0.4 mg on day 5 of pregnancy and might be interrelated with changes of plasma estradiol concentration and sodium and potassium contents in uterine fluid.