Mexiletine administered either intravenously or orally was found to be an effective antiarrhythmic agent. Unfortunately, important adverse effects occurred in approximately half of the patients. Adverse effects which appeared during oral therapy were dose related. Mexiletine controlled lignocaine-resistant ventricular dysrhythmias in more than half of the patients treated. Ventricular tachycardia and ventricular fibrillation occurred at plasma concentrations which were not significantly different from those associated with severe side-effects, suggesting that when mexiletine is used to control these major dysrhythmias the incidence of important side effects will be high.