The ranal microvasculature evidenced by intraarterial injection of silicone rubber performed in 10 cases of acute renal failure from various causes demonstrated definite reduction in filling with patchy deficit in subcapsular cortex, narrowing of preglomerular arterioles, especially at the branching from the interlobular arteries in the same area, relative increase in filling of vas afferens and efferens of juxtamedullary cortex, and of vasa recta in medulla. The same injection figures are observed in acute renal failure in dogs experimentally induced by ligation and clamping of renal arteries, and intraarterial infusion of large quantities of angiotensin II (1000 ng/kg/min). Although the etiological factors of acute renal failure are manifold and the parenchymal damage is varied as well, the vasoconstriction of preglomerular arterioles should be considered as an initial common pathway which continues in subcapsular cortex. The result of present experiments is consistent with the hypothesis that renin and angiotensin participate in the pathogenesis of acute renal failure.