Mexiletine was used in 12 patients. Serum drug levels and the antiarrhythmic effects were studied in a therapeutic protocol comprising an initial intravenous administration (3,5 mg/Kg over 10 mins, then 450 mg in 5 hours) followed by oral administration (200 mg eight hourly). The serum drug levels of Mexiletine were high (1,46 +/- 0,47 microgram/ml) as from the first hour in all patients, and remained constant during intravenous administration (average: 1,59 +/- 0,72 microgram/ml). On oral therapy the average mexiletine level was 1,18 +/- 0,21 microgram/ml with two lows (1,02 microgram/ml) six to eight hours after the ingestion of the 200 mg gelules. Ventricular extrasystoles completely regressed after the initial rapid intravenous injection in 7 out of the 12 patients (58,3 p. 100) and this efficacity was maintained throughout the trial. In two patients with low serum mexiletine levels at the end of oral therapy (less than 1 microgram/ml), ventricular extrasystoles were much less frequent during the intravenous phases but reappeared during oral therapy. The clinical, electrocardiographic and hemodynamic tolerance was good (withdrawn in only one patient). The dose should be adapted to the patient's weight and hemodynamic status. When ineffective, the serum mexiletine level can be estimated and when less than 1 microgram/ml, the dose may be increased, in particular by adjusting oral dosage to 200 mg six hourly.