We have summarized some of the studies containing basic biological data suggesting potential therapeutic utility of the anti-proliferative activity of antiprogestins on uterine tissues. The non-competitive anti-oestrogenic effects of RU486 were examined using oestradiol-treated ovariectomized monkeys given RU486, progesterone or both. The oestradiol-induced luteinizing hormone surge of control animals was abrogated by progesterone and/or RU486. Secretory transformation by progesterone was inhibited by RU486 co-administration. RU486 alone (1 mg/kg) induced endometrial secretory transformation, but higher doses (5 mg/kg) induced inhibited proliferation and secretory activity. Thus, in the presence of progesterone, RU486 is antagonistic but, in its absence, RU486 exhibits endometrial progestational effects at low doses and an anti-proliferative (anti-oestrogenic) effect at higher doses. These data encourage continued evaluation of RU486 as a potential contraceptive agent acting at the pituitary and/or endometrial level. Our study also demonstrates that after physiological oestradiol replacement therapy, oestradiol receptor concentrations rise dramatically following antiprogestin treatment; this effect was dose-dependent.