The effect of synthetic 8-arginine vasopressin (vasopressin) was studied in isolated canine basilar, left circumflex coronary, and femoral arteries of the dog. Vascular rings with and without endothelium were suspended for isometric tension recording in physiological salt solution. The removal of the endothelium was confirmed by the absence of relaxations induced by either thrombin (basilar arteries) or acetylcholine (coronary and femoral arteries). In the basilar artery, vasopressin induced concentration-dependent inhibition of myogenic tone. In basilar and coronary arteries, the hormone caused concentration-dependent relaxations during contractions evoked by prostaglandin F2 alpha. In femoral arteries, vasopressin caused contraction. After removal of the endothelium, the inhibitory responses to vasopressin were abolished in basilar arteries and significantly reduced in left circumflex coronary arteries. The contractions of femoral arteries were not affected by endothelium removal. The V1-vasopressinergic antagonist d(CH2)5Tyr(Me)AVP prevented the inhibitory response to vasopressin, but did not alter endothelium-dependent relaxations of basilar arteries caused by adenosine diphosphate. These results demonstrate that the endothelial cells mediate relaxation induced by vasopressin via specific V1-vasopressinergic receptors.