Activation of the renin-angiotensin system has been implicated as one of the mechanisms involved in acute renal failure. Support of this hypothesis was forwarded by the finding that propranolol, a blocker of renin release, reduced the deleterious effect of ischemia on kidney function. The hypothesis was tested further in these experiments by studying renal blood flow after ischemic injury to rat kidneys with and without propranolol proection. Miniature electromagnetic flow probes measured blood flow after 70 min of total renal artery occlusion. Although brief infusion of propranolol reduced the degree of acute renal failure, it had no effect on renal blood reflow. The data do not support the hypothesis that the renin-angiotensin system plays a role in acute renal failure. Other explanations for the protective effect of propranolol are suggested.