The renin inhibitor H77 and the angiotensin I converting enzyme (ACE) inhibitor captopril were compared in separate experiments with infusion of 5% dextrose as a control for the effects on plasma angiotensin II (Ang II) concentration, arterial pressure and cardiac function, measured by Swan-Ganz catheter, in conscious dogs. The effects of a high dose of H77 (10 mg/kg per h) were similar to those of high-dose captopril (6 mg/kg per h). Both reduced plasma Ang II concentration, systemic vascular resistance and arterial pressure; both increased the heart rate; both increased cardiac output but the change was significant only with captopril; neither affected stroke volume, pulmonary artery pressure or pulmonary vascular resistance; both reduced left and right atrial pressures. The similar pattern of effects for the two inhibitors suggests that the mechanism by which they act is the same--reduction in Ang II--and that the cardiovascular effects of H77 are not a specific action of the peptide that is unrelated to the reduction in plasma Ang II concentrations.