Twenty-three patients with mild heart failure (I-II NYHA classes) on digitalis and diuretics were assigned to the following treatment in a random and double-blind fashion: ibopamine-captopril, ibopamine-placebo, captopril-placebo, and placebo-placebo. The doses of captopril and ibopamine were respectively 25 mg t.i.d. and 100 mg t.i.d. The incremental exercise time (until exhaustion) and the peak VO<sub>2</sub> (oxygen consumption) the indexes of left ventricular function (by echo and nuclear stethoscope) and ventricular arrhythmias (evaluated by prolonged Holier monitoring) were assessed before randomization, at 45 days and at 3 months. Ejection fraction, exercise time, peak VO<sub>2</sub>, ventricular arrhythmias and heart rate (at rest and during exercise) appeared to be equally unaffected by each treatment. Our results show that ibopamine exerts no significant effects on either heart rate or ventricular arrhythmias and that indexes of left ventricular function are not modified by any treatment in mild congestive heart failure.