The effect of acute administration of the angiotensin-converting enzyme inhibitor captopril on arterial pressure, glomerular filtration rate, and renal plasma flow was assessed in 16 patients with severe congestive heart failure. Following administration of captopril, mean arterial pressure (MAP) fell in all cases, whereas effective renal plasma flow increased from 27% to 88% in 10 patients, remained unchanged in 4, and decreased by 60% and 93% in 2 patients in whom MAP fell to 54 and 47 mmHg, respectively. Effective renal plasma flow and glomerular filtration rate values achieved after captopril were both positively correlated with postcaptopril MAP. The results of this study suggest that the renin-angiotensin system plays a major role in the regulation of MAP and the renal vasoconstriction associated with severe congestive heart failure. However, angiotensin blockade may induce a deterioration in renal function in patients in whom arterial pressure falls to markedly low values, thus suggesting an influence in angiotensin in renal autoregulation in these patients.