Multifocal atrial tachycardia (MAT) is a difficult arrhythmia to treat. Pharmacologic treatment is generally disappointing, and successful conversion in a predictable manner is uncommon. To assess the efficacy of metoprolol, a relatively selective beta 1-adrenergic blocking agent, we administered this agent to 11 patients (aged 71.8 +/- 8.3 yr). All patients had serious pulmonary disease. Hypoxia, hypercarbia, acidosis, and electrolyte abnormalities were corrected before the study. Nine patients were receiving theophylline derivatives and six digoxin. Serum levels for both drugs were in the therapeutic range. Four patients had received verapamil without control of MAT. Mean atrial rate before administration of metoprolol was 142.3 +/- 17.2 beat/min and mean ventricular rate was 131.4 +/- 24.3 beat/min. One to 3 h after metoprolol (25 or 50 mg orally), all patients were restored to sinus rhythm, with a mean ventricular rate of 86.9 +/- 6.8 beat/min (p less than .01). Six patients had 3 to 6 premature atrial contractions per minute. No adverse effects were noted, and arterial blood gases before and after therapy were comparable. Five patients expired from their underlying disease and four were continued on metoprolol to maintain sinus rhythm. Metoprolol is effective in the acute and chronic treatment of MAT and may be given to patients with MAT and respiratory failure without serious adverse effects.