The systemic and regional circulatory effects of angiotensin-converting enzyme inhibition were investigated in 30 normal subjects and in 36 patients with severe congestive heart failure. Regional blood flow was measured in individual patient groups. Cardiac index rose and systemic vascular resistance fell in normal subjects after angiotensin-converting enzyme inhibition. In the patients with heart failure, a similar rise in cardiac index and fall in systemic resistance occurred. In addition, right and left ventricular filling pressures decreased. The fall in systemic vascular resistance correlated with plasma renin activity (r = 0.57, p less than or equal to 0.001). Of the regional circulations investigated in normal subjects, only forearm blood flow increased after angiotensin-converting enzyme inhibition. Although over-all there was no change in renal or coronary blood flow, coronary flow dramatically increased in some patients and the increase in flow correlated with plasma renin activity (r = 0.939, p less than or equal to 0.001). In patients with heart failure, forearm, splanchnic, and coronary flow were unaffected by angiotensin-converting enzyme inhibition, whereas renal blood flow estimated from para-aminohippurate clearance increased 60 percent and accounted for 50 percent of the increase in cardiac output seen in these patients. Thus, redistribution of flow occurs in congestive heart failure with a significant reduction in the fraction flow to the kidneys when compared with normal flow. The contribution of the renin-angiotensin system to the regulation of regional blood flow is different in normal subjects and in patients with heart failure. Angiotensin-converting enzyme inhibition augments skeletal flow in normal subjects whereas it increases renal blood flow in patients with heart failure.