Electrophysiologic studies have been performed in 11 patients before and 12 minutes after intravenous administration of 250 mg of mexiletine. All patients had crisis of paroxysmal tachycardia. The cases with conduction defects at any level, as ascertained by surface ECG or basal electrophysiologic recordings, were excluded from treatment. The results obtained do not show significant variations in any of the parameters studied. It is concluded that mexiletine is an antiarrhythmic drug with an action typical of group I of the classification of Vaughan Williams, and that it reduces the maximum velocity of systolic depolarization. In the absence of conduction defects, its behaviour is similar to that of lidocaine or diphenylhydantoin.