We studied, by 48-hour Holter monitoring, 33 patients with chronic stable heart failure (radionuclide ejection fraction less than 35%), complex ventricular arrhythmias and no electrolyte abnormalities, after a period during which they were treated with digoxin and diuretics. Before Holter monitoring blood samples were analyzed for serum concentration of sodium, potassium, magnesium, urea, creatinine, digoxin, aldosterone and for plasmatic renin activity in addition to urinary aldosterone and catecholamines determination. After these investigations in 23 patients, 5-20 mg of enalapril were progressively added to the conventional therapy, while 10 patients continued the previous therapy. After 8 weeks 30 patients were subjected to a second 48-hour Holter monitoring and to the same biochemical and hormonal tests. One patient died and 2 were lost to follow up. Only the enalapril group showed a significant decrease in the number of premature ventricular complexes (PVC) (p less than 0.01), and the frequency of couplets and episodes of ventricular tachycardia (VT) declined significantly (P less than 0.01). In the two groups there were no significant changes in digoxin, sodium, or magnesium, while potassium concentration increased in both groups (p less than 0.01). In the enalapril group heart rate and systolic and diastolic pressure declined significantly (p less than 0.01), and New York Heart Association class (NYHA) improved (p less than 0.001). In the other group there were no significant changes in these parameters. Enalapril caused a significant increase in the plasmatic renin activity (p less than 0.01) and a significant fall of plasma and urinary aldosterone (p less than 0.01; p less than 0.05).