The efficacy of intravenously administered metoprolol, a cardioselective beta-adrenergic blocking agent, was evaluated in the treatment of supraventricular tachyarrhythmias in 16 patients. The arrhythmias that were treated were atrial fibrillation (11 patients), atrial flutter (2 patients), supraventricular tachycardia (2 patients), and multifocal atrial tachycardia (1 patient). Mean dose of metoprolol was 9.5 mg (range: 2-15 mg) administered in one or two separate infusions of up to 7.5 mg each over a cumulative maximum interval of 25 minutes. In the 13 responders (81%), mean ventricular rate decreased from 134 +/- 6 to 106 +/- 7 beats/min 10 minutes after metoprolol administration and was controlled for 40 to 320 minutes without further therapy. Minimum ventricular rate (98 +/- 6 beats/min) was reached 48 minutes after initiation of metoprolol. Metoprolol reduced ventricular rate by greater than 15% (decrease of 26-60 beats/min) in 11 (69%) of 16 patients, including 9 (82%) of 11 patients with atrial fibrillation. In two other patients, one with atrial fibrillation and one with supraventricular tachycardia, ventricular rate was reduced by greater than 12%. Hypotension, occurring in five patients, was the most frequent side effect but was transient and readily managed. Cardioselective beta-adrenergic blockade by metoprolol was rapidly effective in controlling ventricular rate in a majority of patients with supraventricular tachyarrhythmias and may be of particular use in selected patients with chronic obstructive pulmonary disease in whom intravenous beta-adrenergic blockade is indicated. Hypotension is an important potential side effect.