Twenty-four-hour Holter electrocardiographic recordings were used to measure the effects of a converting-enzyme inhibitor, enalapril, given for 12 weeks, on the frequency of cardiac arrhythmias in 10 patients with congestive heart failure (New York Heart Association functional class II to III) receiving maintenance therapy with digoxin and furosemide. Nine patients were given placebo, and both study groups were conducted in a double-blind, parallel manner. The placebo group had no change in the frequency of arrhythmias, whereas enalapril-treated patients showed a significant decrease in the frequency of premature ventricular complexes, ventricular couplets and ventricular tachycardia. A minor, nonsignificant reduction in atrial premature complexes was seen in patients who received enalapril. Compared with placebo patients, those who received enalapril had an increase in plasma potassium levels of 0.33 mmol/liter, a decrease in plasma digoxin, and decreases in pulmonary artery wedge, mean pulmonary artery and right atrial pressures. However, none of these indexes were correlated with the concomitant decline in cardiac arrhythmias. It is concluded that enalapril reduces the frequency of ventricular arrhythmias in congestive heart failure, although the underlying mechanisms are not known.