In a single-blind, placebo-controlled study, esmolol was administered intravenously to 12 patients with chronic atrial fibrillation. Esmolol produced a significant dose-dependent decrease in the ventricular rate without conversion to normal sinus rhythm in any of the patients. For most patients, a correlation was observed between the blood levels of esmolol and reductions in heart rate. There were no significant adverse effects. We conclude that esmolol is an effective and safe agent for the control of heart rate in patients with supraventricular tachycardia.