OBJECTIVE: To evaluate whether xamoterol, a partial agonist, would improve exercise time more than metoprolol in patients with mild to moderate heart failure after a myocardial infarction. DESIGN: Single-centre double blind randomised parallel group comparison of metoprolol 50-100 mg and xamoterol 100-200 mg twice daily. PATIENTS: 210 patients aged 40-80 years (173 men) with clinical evidence of heart failure early after a myocardial infarction. 106 were given metoprolol and 104 xamoterol. MAIN OUTCOME MEASURES: Exercise test results and performance at three months; the exercise test, quality of life, and clinical assessments at baseline (5-7 days after the infarction) and after 3, 6, and 12 months. RESULTS: Exercise time increased at three months by 22% in the metoprolol group and 29% in the xamoterol group, but with no significant difference between the groups. Patients taking xamoterol showed overall non-significantly higher mean values of exercise time achieved with higher heart rates at rest and exercise. Improvements in quality of life, clinical signs of heart failure, and New York Heart Association functional class were seen in both treatment groups over one year, with minor benefits of xamoterol on breathlessness, peripheral oedema, and functional class. Eighteen patients taking metoprolol and 22 taking xamoterol withdrew from the study during one year, with a low mortality, reinfarction rate, and progress of heart failure in both treatment groups. Mean dose from baseline to 3 months was 135 mg metoprolol and 347 mg xamoterol. CONCLUSION: beta 1 Receptor antagonists with or without partial agonist activity are safe to use in mild to moderate heart failure after a myocardial infarction. Exercise tolerance, quality of life, and clinical signs and functional class of heart failure improve, and few patients show deterioration in their condition. Exercise tolerance is no better with xamoterol than metoprolol.