RU 486 is a synthetic 19 norsteroid with a great affinity for progesterone and glucocorticosteroid receptors. The antiprogesterone activity of RU 486 permits interruption of the luteal phase of the menstrual cycle and of early pregnancy, while the antiglucocorticosteroid activity interrupts the feedback mechanisms of cortisol at the hypothalamic-pituitary axis. 11 women aged 18-34 years who were 6-8 weeks pregnant were given daily doses of 200 mg of sensation of lipothymia. Blood pressure and laboratory test results remained normal. The daily increase of beta human chorionic gonadotropin (HCG) and progesterone was stopped by the antiprogesterone. It has been demonstrated in vitro that RU 486 inhibits secretion of beta HCG by cells of trophoblastic origin. 2 weeks after expulsion of the products of conception, the blood levels of beta HCG, progesterone, and estradiol had returned to nonpregnant values and ovarian activity was normal in women not using oral contraceptives. The reason for failure of expulsion in 2 cases was not known. RU 486 was also used to interrupt the luteal phase of normal menstrual cycles in 3 young women. 50 mg/day of RU 486 starting on the 22nd day was sufficient to induce bleeding within 48 hours. Studies using RU 486 50 mg/day beginning on the 10th cycle day indicate that progesterone plays a central and follicular role during the preovulatory period. The antiglucocorticosteroid activity of RU 486 was observed in pregnant women who received the abortifacient dose of 200 mg/day. The 8 a.m. plasma level of ACTH and beta-LPH cortisol remained elevated for thee 4 days of treatment. Nonpregnant women receiving 50 mg of RU 486 for 4 days beginning on the 22nd cycle day had no change in the level of hypophyseal-adrenal hormones.dd Tests in young male volunteers showed that RU 486 amplified circadian rhythms of the hypophysealadrenal axis, affecting specifically the morning hormonal levels without influencing afternoon levels. A test in which RU 486 was used to antagonize the inhibition producedd by dexamethasone during a test of suppression of the hypophyseal-adrenal axis confirmed that the antiglucocorticosteroid effect of RU 486 is produced at the level of the glucocorticosteroid receptors. The antiglucocorticosteroid effect is independent of the antiprogesterone effect. RU 486 may be utilizable in fertility control, to control undesirable effects of steroids in cases of hypercorticism, and perhaps eventually to test the hypophyseal-adrenal axis.