Responses to exogenous norepinephrine (NE), transmural electrical stimulation, 5-hydroxytryρtamine (5-HT) and lysine vasopressin were studied in isolated helical strips of the small artery and vein from the mesoappendix of patients undergoing incidental appendectomy at the time of cholecystectomy. Responses to NE and 5-HT were similar in each vessel. Arterial strips were unresponsive to electrical stimulation and responses to vasopressin were greater than those to NE in this tissue. Venous strips were unresponsive to vasopressin. Relaxation of exogenous NE responses following oil immersion of arterial strips was unaffected by cocaine whereas relaxation of similarly treated venous strips was markedly prolonged. The data suggest: (1) that the artery of human mesoappendix is poorly innervated and (2) that vasopressin is clearly more active on human mesoappendix artery than it is on human mesoappendix vein. The latter observation may help to explain the efficacy of vasopressin infusion in gastrointestinal hemorrhage secondary to portal hypertension.