The antiarrhythmic effect of nadolol, a long-acting, nonselective beta antagonist without intrinsic sympathomimetic or membrane-stabilizing properties, was evaluated in 36 patients with ventricular dysrhythmias as determined by three baseline 24-hour Holter recordings at a time when subjects were receiving placebo. Nadolol was administered once daily at a dose of 40 to 80 mg and increased at weekly intervals to a maximum daily dose of 640 mg. Thereafter the drug was stopped gradually and placebo was given again for a period of 2 weeks. Nadolol was effective in reducing premature ventricular contractions (PVCs) in 17 of 36 patients (48%), in reducing ventricular couplets in 24 of 27 patients (89%), and in reducing nonsustained runs of ventricular tachycardia in all 13 subjects. Serum nadolol levels obtained at dosages resulting in a 75% reduction in PVCs varied from 58 to 853 ng/ml. In the majority of the subjects studied, a nadolol dosage of 160 mg/day or less was effective for arrhythmia suppression.