RU-486 is a recently synthesized steroid with anti-progesterone and anti-glucocorticoid properties. Its direct anti-progesterone action on the uterus is believed to be the basis for its ability to induce menstruation and early abortion. RU-486 likely antagonizes progesterone action on the uterus progesterone receptor. We have studied the binding of RU-486 to rat ovary steroid receptors in order to learn whether this interesting synthetic compound binds to ovary steroid receptors and thus learn if this antagonist can be used to better define the mechanism(s) of steroid actions on the ovary. Ovaries from estrogen stimulated hypophysectomized immature female rats were homogenized in buffer. The 100,000 x g supernatant was incubated with tritiated steroid agonists (R5020, dexamethasone, or testosterone) alone or with unlabeled steroids including RU-486. Receptor bound, and free steroid, were separated by Sephadex G-200 columns. The relative abilities of the various tested steroids to bind to the rat ovary progesterone receptor were: RU-486 greater than or equal to R5020 greater than progesterone. RU-486 bound to the ovary glucocorticoid receptor with an affinity equal to that of dexamethasone. The affinity of RU-486 for the rat ovary androgen receptor was only about 9% that of testosterone. Thus, RU-486 binds with very high affinity to the rat ovary progesterone and glucocorticoid receptors. This steroid receptor antagonist offers a new tool by which the mechanism(s) of action of these steroids on the ovary can be tested.