To further investigate the role of opioids in the regulation of the pituitary-adrenal axis we studied the effect of morphine and naloxone on human corticotropin-releasing hormone (hCRH)-induced ACTH, immunoreactive (ir) β-endorphine, and cortisol release in normal subjects. Protocols: 1. 30 mg of a slow-release preparation of morphine or placebo was given orally 3 h prior to administration of hCRH (0.1 mg iv) (N = 7). 2. Naloxone (4 mg as bolus iv) or placebo was given 5 min prior to hCRH (N = 7). 3. Naloxone (4 mg iv as bolus followed by a continuous infusion of 6 mg over 75 min) or placebo was started 15 min prior to hCRH (N = 6). hCRH was injected at 11.00 h (protocol 1, 2) or at 17.00 h (protocol 3). Oral morphine not only suppressed basal hormone levels (P <0.02), but also the peak response to hCRH compared with placebo (cortisol: 270 ± 50 vs 559 ± 80 nmol/l; ACTH: 5.1 ± 1.5 vs 13.1 ± 2.7 pmol/l; ir β-endorphin: 48.5 ± 8.7 vs 88 ± 14 pmol/l; mean ± sem, P <0.02). Similarly, the maximum incremental changes and the area under the curve were significantly reduced for all three hormones compared with placebo (P < 0.05). After 4 mg of naloxone in the morning, no significant hormonal changes in response to hCRH were observed. However, 10 mg of naloxone in the afternoon led to higher maximum hormone concentrations in response to hCRH compared with placebo (cortisol: 636 ± 30 vs 437 ± 63 nmol/l; ACTH: 19.6 ± 4.4 vs 8.7 ± 1.1 pmol/l; ir β-endorphin: 180 ± 44 vs 94 ± 18 pmol/l, P <0.05). The effect of high-dose naloxone on the hCRH-induced hormone release alone supports the concept of a physiologically significant inhibition of the ACTH release by endogenous opioids via receptors of relative naloxone resistance (δ- or κ-receptors) located at the pituitary level. The μ-agonist morphine may act at suprahypophyseal sites by inhibition of CRH potentiating factors.