Cardiac rehabilitation after a myocardial infarction has been shown to improve exercise capacity. Beta blockade has been shown to be effective in treating angina and reducing mortality, but studies are controversial as to whether beta-blockade therapy attenuates the effects of training. We attempted to study the effects of beta blockade (metoprolol) on the response to training in patients enrolled in a cardiac rehabilitation program after an uncomplicated myocardial infarction. We studied 27 patients with a recent uncomplicated myocardial infarction who were subdivided in two groups: Group 1 (13 patients) not taking a beta blocker, and Group 2 (14 patients) taking metoprolol (mean 142 +/- 57 mg daily). All patients underwent a maximal cardiopulmonary exercise test before and after a 3-month training program. The training intensity was designed to approximate the ventilatory threshold. Results showed an increase in peak VO2 in both Group 1 (27%, p < 0.01) and Group 2 (33%, p < 0.001), and an increase in VO2 at the ventilatory threshold (39% in Group 1 and 28% in Group 2, p < 0.01). The mean changes in exercise capacity were not different between groups. It was concluded that metoprolol did not influence the beneficial effects of a cardiac rehabilitation program in postmyocardial infarction patients.