Progesterone receptor antagonists have been developed by substitutions at the 11-beta and 17 side-chain positions of the progestagen norethisterone. The most studied progesterone receptor antagonists are mifepristone (Mifegyne; Roussel-UCLAF; RU486) and ZK98734 and ZK98299 (Schering AG). These compounds bind avidly to the progesterone receptor and glucocorticoid receptor but have essentially no binding to the mineralocortocoid, oestrogen or androgen receptors. Mifepristone also binds avidly to albumin, resulting in a half-life of approximately 24 h after oral administration. Progesterone receptor antagonists can induce menstruation by a direct action upon the endometrium. They have also been shown to exert weak progesterone agonist actions in certain circumstances and to modulate pituitary hormone secretion by antagonizing the feedback actions of progesterone. Moreover, they release prostaglandin F2 alpha and E2 from human endometrium or early pregnancy decidua and reduce the metabolism of these eicosanoids. Clinically, progesterone receptor antagonists have been used in trials of menstrual regulation, abortion and induction of labour, and during treatment of breast or ovarian cancer, some forms of hypertension and meningioma. Progesterone receptor antagonists have been administered to approximately 70,000 women in 18 countries as medical abortifacients. They have been proven, especially when combined with prostaglandin analogues, to be as effective as surgical methods of termination of pregnancy. Progesterone receptor antagonists have focussed international attention on menstrual regulation, abortion and the rights of women to regulate their fertility.