RU 486 is a synthetic steroid hormone antagonist which acts at the receptor level. It has both intrinsic anti-progesterone and antiglucocorticosteroid properties in animals. We investigated the antiglucocorticosteroid activity in humans by evaluating the pituitary-adrenal response to RU 486 in men and in pregnant and nonpregnant women. In non-pregnant women, RU 486 (approximately equal to 1 mg/kg of body weight per day) produced an interruption of the luteal phase without affecting the pituitary-adrenal axis, thereby indicating a more potent anti-progesterone than antiglucorticosteroid effect. In the course of pregnancy interruption by RU 486 (approximately equal to 4 mg/kg per day), there was a significant increase in plasma corticotropin, beta-lipotropin, and cortisol concentrations. In normal men, RU 486 administration led to a dose-dependent stimulation of plasma corticotropin, beta-endorphin, and cortisol. This disinhibition of the pituitary-adrenal axis was only observed during the morning hours of the circadian rhythm. When administered concomitantly with 1 mg of dexamethasone at midnight, 6 mg of RU 486 per kg completely suppressed the dexamethasone inhibitory effect on the pituitary-adrenal axis. These results indicate that RU 486 is an antiglucocorticosteroid that disrupts the negative pituitary feedback of both the morning cortisol rise and administered dexamethasone. Furthermore, they demonstrate the possibility of optimizing the anti-progestational effect of the compound and its potential use for human fertility control by modifying the dose and the time of administration of the drug and thereby minimizing the antiglucocorticosteroid effect.