Captopril (SQ 14,225), an oral angiotensin converting-enzyme inhibitor, was administered to 11 patients with severe congestive heart failure (CHF). Peak effect was observed at 1.5 hours after administration. At peak effect right atrial pressure fell from 3.4 to 0.0 mm Hg, pulmonary capillary wedge pressure (PCW) fell from 22.7 to 12.3 mm Hg, mean arterial pressure (MAP) fell from 79.5 to 62.1 mm Hg, systemic vascular resistance (SVR) fell from 1989 to 1370 dyn-sec-cm-5, pulmonary vascular resistance fell from 843 to 523 dyn-sec-cm-5, and cardiac index (CI) rose from 1.96 to 2.43 l/min/m2. These were all statistically significant. Control plasma renin activity (PRA) was elevated (25.9 ng/ml/hr) and correlated with resting PCW (r=0.65). The acute hemodynamic response was related to PRA: a fall in MAP (r=0.74), a fall in PCW (r=0.80), a fall in SVR (r=0.45) and a rise in CI (r=0.45). Eight patients were placed on chronic captopril therapy. After 2 or more months, their exercise time was significantly increased, from 6.8 to 11.7 minutes. Their cardiothoracic ratios showed a significant decrease, from 0.55 to 0.52, and most patients reported symptomatic improvement. Chronic response was not predicted by acute hemodynamic response. Captopril is therefore a vasodilator with both arterial and venous effects that are at least partially caused by inhibition of the renin-angiotensin system. It may be useful for the treatment of CHF.