Experiments were performed to determine the site of arginine vasopressin (AVP)-induced vascular resistance adjustments that result in decreases in papillary blood flow. Simultaneous measurements of luminal diameter and centerline erythrocyte velocity allowed estimation of single-vessel blood flow in in vitro blood-perfused juxtamedullary nephrons. AVP (0.1-1,000 pM) caused concentration-dependent reductions in outer medullary descending vasa recta (OMDVR) blood flow (10 pM) without altering OMDVR diameter. Afferent arteriolar diameter was significantly decreased by 1 pM AVP, whereas arcuate artery diameter was decreased by 100 pM AVP. Increasing the concentration of AVP to 1,000 pM resulted in significant reductions of vessel diameter in interlobular arteries, distal afferent arterioles, and efferent arterioles. The effects of AVP to decrease afferent arteriolar diameter and blood flow were blocked in the presence of V1 receptor antagonist. These data indicate that afferent arterioles exhibit the greatest sensitivity to the vasoconstrictor effects of AVP, whereas OMDVR appear insensitive. We conclude that the change in OMDVR blood flow produced by AVP is not due to a direct effect of the peptide on this vascular segment but to its effect on upstream vessels.