Mexiletine is an antiarrhythmic agent, structurally similar to lidocaine, but useful orally as well as intravenously. The safety and efficacy of mexiletine was compared with quinidine in a double-blind, parallel, randomized study of patients with chronic ventricular ectopia of various causes. Mexiletine was statistically more effective (P less than .05) than quinidine, though the total number of patients studied was small. Like quinidine, side effects limited the usefulness of mexiletine in certain patients, but these side effects are quite distinct from those of quinidine and other type I antiarrhythmic drugs. Mexiletine should be a useful addition to the armamentarium of antiarrhythmic drugs when it is released for clinical use in the United States.