The factors responsible for the development of hypertension during chronic activation of intrarenal V₁ receptors are unknown. We therefore tested whether medullary interstitial infusion of the selective V₁-receptor agonist [Phe²,Ile³,Orn⁸]vasopressin (V₁ agonist) influences renal antihypertensive mechanisms initiated by increased renal perfusion pressure (RPP). In intact anesthetized rabbits, the V₁ agonist (10 ng · kg¹ · min¹) reduced medullary perfusion by 36 ± 7%, whereas cortical perfusion was reduced by only 14 ± 2%. An extracorporeal circuit was used to increase RPP in a stepwise manner from 65 to 85, 110, 130, and 160 mmHg for consecutive 20-min periods. Increased RPP reduced mean arterial pressure by 35 ± 8% in vehicle-treated rabbits, but by only 10 ± 3% in V₁ agonist-treated rabbits. Simultaneously, pressure-diuresis-natriuresis was induced; urine flow and sodium excretion increased similarly in the two groups of rabbits, but hematocrit did not change. We suggest that the depressor response to increased RPP is mainly due to release of a putative renal medullary depressor hormone (RMDH). Suppression of the release and/or actions of RMDH may therefore contribute to the hypertensive effect of chronic V₁ receptor activation.