In this study, the use of enalapril and captopril is compared in the treatment of congestive heart failure. Although both drugs act on the renin-angiotensin system via converting enzyme inhibition, their different chemical structures may dispose them to different pharmacologic and physiologic activity. Both drugs exert a vasodilator effect, with reduction of left and right ventricular filling pressures and aortic impedance. In short-term hemodynamic studies, the onset of action and peak effect are earlier with captopril. Enalapril has a much more gradual onset and longer duration of action. Both drugs have a shallow dose-response curve and both produce comparable hormonal changes: an increase in plasma renin activity and a decrease in aldosterone levels. Captopril also increases prostaglandin production. Long-term efficacy trials have demonstrated symptomatic improvement in patients given captopril and those receiving enalapril who were also receiving digitalis and diuretics. Baseline hemodynamics may not predict long-term improvement. There are few adverse effects for the two drugs, but their incidences differ, suggesting a relationship to chemical structure. Recent studies in congestive heart failure suggest a reduction in mortality with various drug regimens.