We measured cardiac output (CO), renal blood flow (RBF), renal plasma flow (RPF), and afferent arteriolar diameter by the microsphere method, and inulin clearance (GFR) simultaneously in awake rats given an infusion of exogenous angiotensin II (100 ng/min per kg). Angiotensin II did not affect the CO, whereas both RBF and RPF decreased significantly in rats infused with angiotensin II (RBF, 2.88 ± 0.11, mean ±SE; RPF, 1.78 ± 0.09,n= 6) when compared to control animals given saline (RBF, 4.58 ± 0.31; RPF, 2.80 ± 0.24 ml/min per 100 g,n= 6, bothP< 0.005, respectively). Both mean arterial pressure (MAP) and renal vascular resistance (RVR) were significantly higher in rats infused with antiogensin II than in controls. The decrease in GFR did not parallel the reduction of RPF in rats infused with angiotensin II as reflected by their higher values of mean filtration fraction (41.1 ± 1.6%,n= 6) than that of controls (33.7 ± 2.4%,n= 6,P< 0.05). Despite significant elevations of MAP and RVR in rats infused with angiotensin II, their mean afferent arteriolar diameter (19.6 ± 0.24 /im) was not different from that of controls (20.1 ± 0.39 /im). We conclude that angiotensin II preferentially acts at the site of postglomerular vasculature but not at the afferent arteriole. Circ Res 46: 646-650, 1980