The acute hemodynamic, hormonal, and pharmacokinetic responses to the oral angiotensin-converting enzyme (ACE) inhibitor spirapril were studied in 15 patients with moderate to severe congestive heart failure in a baseline controlled dose-ranging study. Doses of 0.3, 1.0, 1.5, 3.125, and 6.25 mg were investigated for 24 h in three groups of five patients each. All doses demonstrated a significant reduction in serum ACE, even after 24 h. Significant reductions in mean arterial pressure, systemic vascular resistance, and pulmonary capillary wedge pressure were observed with doses greater than 1.0 mg spirapril. Maximal significant hemodynamic effects occurred approximately 4-6 h after drug administration. The plasma concentrations of spirapril and its metabolite spiraprilate were dose-dependent. After administration of spirapril, the quick rise to the peak level of spiraprilate suggests rapid metabolism of spirapril into spiraprilate and a slow elimination of this metabolite. No severe hypotension or other serious side effects occurred in the patients studied. The results indicate that spirapril may be expected to be an effective drug in the treatment of congestive heart failure. From our findings we conclude that 1.5 mg spirapril is an optimal starting dose in patients with moderate to severe congestive heart failure.