Angiotensin-converting enzyme inhibitors are effective vasodilators in the treatment of congestive heart failure. Enalapril, a new angiotensin-converting enzyme inhibitor, or placebo, in addition to digoxin and diuretic drugs, were given to 17 patients with chronic congestive heart failure who were followed up for 12 weeks. In random double-blind fashion, nine patients received enalapril and eight received placebo. Cardiac dimensions and function improved slightly but insignificantly in both groups. Treadmill exercise duration increased from a mean value (+/- standard deviation) of 9.1 +/- 3.2 to 12.0 +/- 3.5 minutes during enalapril administration (p less than 0.025) and was unchanged during placebo administration (10.1 +/- 3.7 versus 11.1 +/- 5.2 minutes). Maximal oxygen consumption also increased during enalapril therapy (15.8 +/- 3.4 to 18.4 +/- 4.4 ml/min per kg, p less than 0.05) and remained unchanged during placebo treatment (16.0 +/- 6.4 versus 17.0 +/- 4.6 ml/min per kg). Clinical functional class (Yale scale) improved 3.1 +/- 1.9 points (p less than 0.01) during enalapril treatment but not during placebo treatment (0.8 +/- 3.5 points, no significant difference). No significant side effects were observed. Thus, enalapril appears to be a clinically effective and useful new angiotensin-converting enzyme inhibitor for the management of chronic congestive heart failure.