We used metoprolol orally or iv in 25 patients with multifocal atrial tachycardia that was complicating severe cardiopulmonary illness, and we observed its effect on heart rate and rhythm, BP, and arterial blood gases. All patients showed a dramatic slowing of heart rate, averaging 54.0 +/- 4.0 beat/min (p less than .001), with only small and transient fall in BP. Seventeen (68%) patients experienced conversion to sinus rhythm, 11 immediately, and six more within 3 h. Time to response averaged 5.1 h with the oral form but was less than 10 min when the drug was given iv. The mean oral dose required was 32.5 mg, and the mean iv dose was 6.5 mg. No patient experienced clinically apparent hemodynamic or respiratory deterioration as a result of metoprolol administration. While arterial pH and PaCO2 were unaltered by metoprolol, mean PaO2 increased by 12.2 +/- 5.8 torr (p less than .05), the increase being greatest in patients with more severe gas-exchange derangement. We conclude that metoprolol is effective in the management of multifocal atrial tachycardia.