The inhibitory action of oxytocin (OT) on adrenocorticotropin (ACTH) secretion has been disputed. Thus we evaluated the effect of exogenous OT on the elevated blood ACTH levels in normal human subjects. Metyrapone, a blocker of cortisol secretion, was given to enhance ACTH release. This experimental model was chosen because metyrapone-induced ACTH activation depends on diminution of the negative feed-back of cortisol, which is an important physiological mechanism in the control of ACTH secretion. A striking decline in plasma cortisol levels and a 10-fold rise in the mean plasma ACTH concentration was observed within 20 h after the beginning of metyrapone treatment (750 mg orally every 4 h). The administration of OT (2 IU as a i.v. bolus plus 4 IU infused in 2 h) significantly reduced the metyrapone-induced plasma ACTH rise. Since the effect of OT was evident when ACTH secretion was enhanced by a reduced cortisol-dependent negative feed-back, confirmation of the inhibitory action of OT on the ACTH secretory system in man is provided.