Adequate control of ventricular rate in patients with chronic atrial fibrillation (AF) may not be achieved with digitalis alone. In the present study the additional effect of two different doses (50 mg and 50 +/- 50 mg) of oral metoprolol, a new selective beta-blocking agent, on ventricular rate in patients with longstanding AF has been studied. A decrease in the mean ventricular rate during rest and during exercise at various work loads was observed after both doses. The effect was more pronounced at the highest work load of 80 W, both after 50 mg (p less than 0.002) and after 50 +/- 50 mg (p less than 0.01) of the drug. A high initial heart rate at rest or during exercise was reduced more by the drug than a lower one. Exercise tolerance was reduced in 2 patients. These results suggest that patients with AF in whom satisfactory control of heart rate cannot be achieved with digitalis alone may benefit from addition of individualized metoprolol therapy.