Effects of an anti-progesterone substance, RU 486, on the luteal function were studied in pseudopregnant rats. The 2mg/kg injection RU 486 between day 2 and 9 of pseudopregnancy could not affect on the ovarian function until day 8 (the ascending and the plateau phases) of pseudopregnancy, but did induce luteolysis on day 10 (the descending phase) of pseudopregnancy. On the other hand, the lOmg/kg injection of RU 486 rather induced ovulation, formed new corpora lutea resulted in the increase in serum progesterone levels on day 10 of pseudo- pregnancy. These results indicate that progesterone is an important factor to maintain the pseudopregnancy in rats.