Long-term treatment with beta-adrenergic blocking drugs has been shown to induce clinical amelioration in patients with chronic heart failure. However, the efficacy of these agents has not been consistent, and the mechanisms of their beneficial effects remain to be established. The present study evaluated the influence of oral metoprolol on symptoms and exercise tolerance of patients with idiopathic dilated cardiomyopathy (3 women and 9 men, left ventricular ejection fraction < 0.45, NYHA functional class II or III). One patient did not tolerate metoprolol, whereas 11 patients terminated the study. After 6 months of beta-blocking therapy, detectable improvements of symptoms (NYHA class and questionnaire-derived symptom score) were observed in 6 patients. Six patients reported an increase in functional capacity [oxygen consumption at peak exercise (VO2p) during cardiopulmonary exercise test]. For the whole group, no significant changes in symptoms and exercise tolerance were detected. During exercise, oxygen pulse (VO2/heart rate) and VO2/RPP (VO2/heart rate/systolic pressure) were significantly increased after 6 months on metoprolol (+35, 9% and +27.1%, respectively; both p < 0.01 vs baseline). In conclusion, beta-blocking therapy was well tolerated by the majority of patients, some of which reported improvement of symptoms and functional capacity. The observed increase on oxygen pulse and Vo2/RPP suggests that beta-blockade may reduce myocardial oxygen requirements in proportion to cardiac work. An increase in the energy available to myocardial cells for synthetic and reparative processes may thus account, at least in part, for the beneficial influence of long-term beta-blockade in heart failure patients.