Alprenolol (Aptine), a new beta-adrenergic blocking agent, was administered intravenously on 25 occasions to 23 patients with atrial arrhythmias. Significant slowing of ventricular rate was achieved in 20 patients by decreasing atrioventricular conduction in atrial fibrillation and flutter, and by slowing of the rate of ectopic impulse formation in paroxysmal supraventricular tachycardia. Four patients with the latter arrhythmia reverted to sinus rhythm following alprenolol administration. In four patients with atrial fibrillation, the chronotropic effect of isoproterenol was substantially abolished after treatment with alprenolol. Alprenolol appears to be a safe and effective beta-adrenergic blocking agent in the treatment of atrial arrhythmias.