A possible interaction between synthetic ovine corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) was tested in anesthetized and in freely moving rats. In animals whose endogenous CRF release was blocked by chlorpromazine-morphine-nembutal, AVP elicited a significantly lower maximum ACTH response than did CRF, whereas the concomitant injection of both peptides resulted in a marked potentiation of CRF-induced ACTH secretion. In freely moving rats, AVP was more potent than CRF (on an equimolar basis) in elevating plasma ACTH levels. Since immunoneutralization of endogenous CRF by the administration of anti-CRF serum significantly reduced ACTH release due to injected AVP in these animals, we suggest that at least part of the AVP-induced ACTH secretion observed in nonanesthetized rats may be due to a potentiation of endogenous CRF by exogenously administered AVP. These data support previous reports of an in vitro synergism between CRF and vasopressin, and emphasize the complex role played by the interaction of these two peptides on ACTH release in vivo.