The antiarrhythmic efficacy of propranolol was evaluated in 32 patients with chronic high frequency ventricular arrhythmias in a placebo-controlled protocol. After a placebo control period, propranolol was begun and the dosage increased sequentially until arrhythmia suppression was achieved, side effects appeared, or a maximum dosage of 960 mg/day was reached. Computerized analysis of ambulatory recordings was used to quantify the arrhythmias. Twenty-four patients had 70-100% arrhythmia suppression at plasma levels ranging from 12-1100 ng/ml (end of dosing interval). Eight patients in this group had frequent episodes of ventricular tachycardia that were totally suppressed at or below the dosage that produced ≤70% suppression of ventricular ectopic depolarizations (VEDs). A biphasic dose-response curve was seen in five patients who responded with a decrease in arrhythmia frequency in the lower ranges of dosages but had increased frequency of ectopic rhythms as the dosage was increased above the optimal level. Only one-third of patients responded at dosages ≤160 mg/day. However, with dosages of 200-640 mg/day, an additional 40% responded. Propranolol appears to control ventricular arrhythmias safely and effectively in many patients. The finding that the antiarrhythmic effect in many patients required plasma concentrations greater than those that produce substantial f-adrenergic blockade raises a question whether blockade of cardiac d receptors can directly account for all of the antiarrhythmic actions of propranolol.