The effects of i.v. captopril on regional blood flow (radioactive microspheres, 15 +/- 5 micron), hemodynamics and maximal oxygen consumption were evaluated in conscious rats with congestive heart failure due to large myocardial infarction (n = 9, infarct size 39.5 +/- 2% of left ventricle) and compared to data obtained from rats subjected to sham surgical procedures (n = 8). In both groups data were obtained at rest and during submaximal treadmill exercise during alternate infusion of captopril and saline. In the congestive heart failure group captopril reduced systemic vascular resistance, mean arterial pressure and left ventricular systolic pressure (P less than .05 each). Blood flow to the renal, gastrointestinal and coronary circulations was reduced in the heart failure group treated with saline vehicle. Flow to the renal and gastrointestinal beds of heart failure animals was enhanced to values similar to those observed in sham animals during captopril treatment. Left ventricular coronary flow was also increased significantly by captopril in both sham and heart failure animals. The most prominent effects of captopril occurred in the renal circulation of the heart failure group in which blood flow increased by 55%. Blood flow to skeletal muscle and skin was unchanged by captopril both in sham and heart failure animals at rest and during exercise. Maximal oxygen consumption was not affected by captopril treatment. Thus, captopril induced a differential pattern of vasodilation with the greatest effect in the renal bed and a less intensive effect in the gastrointestinal and coronary beds. The unchanged flow to skeletal muscle may explain the failure of captopril to improve exercise capacity after short-term administration.