This study compares the activity of two corticotropin-releasing hormone (CRH) antagonists, astressin B and antalarmin, in rhesus monkeys. Astressin B was found to be more potent than antalarmin in abolishing the CRH-stimulated adrenocorticotropin (ACTH) and cortisol responses. The differences between the two may be due to non-CRH receptor-mediated effects of antalarmin or to a complex interaction of antalarmin's effects at both central and peripheral CRH receptors.