In anesthetized dogs the intravenous infusion of arginine vasotocin, 25 mU/min, or vasopressin, 10 mU/min, inhibited renin secretion when control secretion rates were normal or when they were elevated by complete ureteral occlusion. Vasotocin, 5 mU/min, or vasopressin, 1 mU/min, was ineffective. Direct intraarterial infusion of vasotocin, 5 mU/min, into the right kidney inhibited renin secretion by the right kidney only. Glomerular filtration rate and renal plasma flow were not changed, and mean arterial pressure was either not changed or was decreased. Vasotocin caused no change in urine flow but did produce a significant natriuresis secondary to decreased tubular reabsorption. The physiological implications of these polypeptide-renal relationships are discussed.