Sixty unanesthetized patients with 74 cardiac arrhythmias received lidocaine intravenously in an average dose of 1.5 mg/kg in 30 seconds. Suppression or termination of ventricular arrhythmias occurred in more than 80% of the 33 patients whether they had an acute myocardial infarction, were receiving digitalis, or were not receiving digitalis or digitalis toxic. Only 15% (six out of 41 patients) of the supraventricular arrhythmias were affected. Minimal side effects, mainly drowsiness, appeared in 47% of the patients and lasted less than five minutes. Blood pressure changes were not significant and no hypotension or convulsions occurred. Continous infusion of lidocaine (1 to 3.5 mg/min) for up to five days, without side effects, suppressed ventricular arrhythmias in all 18 patients treated. Lidocaine appears effective in suppressing or terminating ventricular arrhythmias occurring in a variety of clinical conditions. It is only occasionally effective in atrial arrhythmias.