Experiments were performed in anesthetized renal-denervated rats to determine the contribution of renal medullary vasopressin V1 and V2 receptor stimulation in the regulation of renal medullary blood flow. Renal medullary interstitial infusion of the selective V1 agonist [Phe2,Ile3,Orn8]vasopressin (2 ng.kg-1.min-1) significantly decreased outer medullary blood flow by 15% and inner medullary blood flow by 35%, as measured with implanted optical fibers for laser-Doppler flowmetry. Medullary interstitial infusion of equimolar doses of arginine vasopressin (AVP) also decreased outer medullary blood flow by 15% but decreased inner medullary blood flow by only 17%, a decrease significantly less than that during the infusion of the V1 agonist. These results were confirmed in videomicroscopy experiments on the exposed papilla, which demonstrated that the V1 agonist and AVP decreased descending and ascending vasa recta capillary red blood cell velocity and calculated blood flow, with greater decreases during infusion of the V1 agonist. In further laser-Doppler flowmetry studies, stimulation of V2 receptors by medullary interstitial infusion of 1-desamino-8-D-arginine vasopressin (2 ng.kg-1.min-1) or AVP in rats pretreated with the vasopressin V1 receptor antagonist d(CH2)5[Tyr(Me)2,Ala-NH2]AVP increased renal medullary blood flow by 16 +/- 3 and 27 +/- 8%, respectively. The present experiments indicate that vasopressin V1 receptor stimulation serves to decrease renal medullary blood flow while V2 receptor stimulation appears to increase renal medullary blood flow; however, the net effect of AVP is to decrease renal medullary blood flow.