A comparison was made of the vascular actions of two hormones having a renal site of action, angiotensin II and vasopressin, using laser Doppler flowmetry to measure perfusion of the cortical and papillary regions of the kidney. Angiotensin II infusion caused dose-related increases in blood pressure and reductions in cortical perfusion, the latter responses being potentiated in the presence of the converting enzyme inhibitor, cilazapril. However, angiotensin II had no effect on papillary perfusion either before or following cilazapril. The reasons for this differing vasoconstrictor ability of angiotensin II at the cortex and papilla are unclear, but it could be due to medullary generation of prostaglandin or bradykinin. Administration of equipressor doses of vasopressin caused graded reductions in both cortical and papillary perfusions, and subsequent cilazapril significantly enhanced the papillary responses. This study demonstrates that the regulation of blood flow through the different regions of the kidney can be differentially regulated by the peptide hormones angiotensin II and vasopressin.