In this study 24-h Holter electrocardiographic recordings were used to measure the effects of an angiotensin converting enzyme inhibitor, enalapril given for 4 weeks, on the frequency of cardiac arrhythmias in 24 patients (14 patients had enalapril, 30 patients had placebo) with congestive heart failure (New York Heart Association Functional Class 3) receiving maintenance therapy with digoxin and furosemide. Although the placebo group had no change in the frequence of arrhythmias, enalapril-treated patients showed significant decrease in the frequency of premature ventricular complexes couplet, bigemine VPS and ventricular tachycardia. Moreover, it was observed that six cases of atrial fibrillation returned to sinus rhythm. During enalapril treatment, some patients experienced increased serum potassium levels, but there was no change in serum digoxin levels. We also observed echocardiographic improvement in left ventricular function as well as clinical symptoms of congestive heart failure. Finally we observed that there was an antiarrhythmic effect of enalapril in congestive heart failure. We thought that the antiarrhythmic effect of enalapril in congestive heart failure was probably due to hemodynamic improvement.