25 patients with recent myocardial infarction and ventricular extrasystole occurring either frequently or in bursts were treated by intravenous and then oral mexiletine. Side-effects occurred during the loading dose in 6 patients, then disappeared without sequel, even while treatment was continued in 2 of these patients, or after it was stopped in the remaining 4 (on account of the severity of the side-effects). Rhythm disorders disappeared under mexiletine in 14 patients as soon as the loading dose was administered; its effectiveness was maintained during intravenous perfusion and after the switch to oral administration. Failure occurred in cases of extensive infarction and/or cardiac insufficiency. Mortality, which was zero in those cases in which mexiletine was effective, reached 45% in the failure group. In cases in which mexiletine proved a failure, lidocaine was also ineffective. In one case in which mexiletine was effective, but was stopped because of side-effects, lidocaine proved ineffective. The efficacy of mexiletine is therefore at least equal to that of lidocaine and is maintained after the switch to oral administration.