The acute hemodynamic and hormonal effects of the oral angiotensin-converting enzyme (ACE) inhibitor lisinopril (MK-521) were assessed over a period of 96 hours in 12 patients with heart failure. This compound is the lysine analogue of enalaprilat (MK-422), is biologically active following absorption, and is cleared via the urine without any known metabolic transformation. Single doses of lisinopril, ranging from 1.25 mg to 10 mg, were administered on days 1 and 3, each followed by 48 hours of intensive hemodynamic observation. Across all doses, maximal reductions in mean arterial pressure (17.2%), mean pulmonary capillary wedge pressure (28%), and systemic vascular resistance (25.6%) were observed compared to baseline values. No significant changes in heart rate were recorded. Arterial blood was sampled at frequent intervals for angiotensin II, ACE activity, plasma renin activity, renin substrate, plasma aldosterone, and serum drug levels. Right atrial blood was sampled simultaneously for angiotensin I, thus permitting assessment of the degree of pulmonary conversion to angiotensin II. The results indicate potent inhibition of the renin-angiotensin-aldosterone system along with hemodynamic efficacy over a period exceeding 24 hours. Frequent clinical follow-up on long-term chronic therapy has revealed no adverse experience.