Patients with acute or chronic heart disease may have limited exercise capacity if they have reduced left ventricular function. Indexes of reduced left ventriuclar function during exercise are a predictor of subsequent survival although left ventricular ejection fraction is a poor predictor of physical endurance. We evaluated the effect of captopril on physical endurance in 48 males with left ventricular dysfunction following myocardial infarction. On the 7th day following myocardial infarction patients were randomized to either captopril 50 mg daily or corresponding placebo. Patients were followed up by means of serial echocardiography and exercise stress testing for a period of 180 days. Exercise capacity was significantly improved in the captopril group. Changes in exercise capacity were significantly correlated to changes in left ventricular (LV) volumes and compliance. The beneficial effect of captopril on exercise capacity was probably mediated via improvement in LV performance and compliance.