Patients with New York Heart Association functional class II or III heart failure stabilized on furosemide therapy were entered into a randomized controlled trial comparing enalapril (n = 72) and digoxin (n = 73). End points were clinical outcome, treadmill exercise capacity and echocardiographic left ventricular dimensions. Improvement in clinical outcome was defined as a reduction of at least one functional class or withdrawal because of an adverse clinical event. After 4 weeks, 13 patients receiving enalapril showed improvement, 55 had no change and 9 manifested deterioration compared with 7, 49 and 17, respectively, in the digoxin group (p less than 0.01). After 14 weeks, 13 patients receiving enalapril showed improvement, 50 had no change and 9 manifested deterioration, compared with 14, 37 and 22, respectively, in the digoxin group (p less than 0.025). More patients in the digoxin group were withdrawn because of an adverse clinical event (p less than 0.05). Exercise time and percent fractional shortening improved in both groups (p less than 0.001 and less than 0.05, respectively), with no significant difference between groups (p greater than 0.50). Both rate-pressure product and subjectively evaluated exertion during submaximal exercise were reduced only in the enalapril group. Although the majority of patients in both groups did well, those receiving enalapril experienced fewer adverse clinical events and had less fatigue during submaximal exercise.