Sustained release procainamide tablets were administered to 34 patients 48 hours after the onset of an acute myocardial infarct. Therapeutic blood levels of procainamide in the range of 4 to 8 mug/ml were consistently achieved using an 8-hourly maintenance dose of 1.5g after an initial loading dose of 2g. In contrast conventional procainamide capsules administered to 21 comparable patients repeatedly failed to produce plasma concentrations in the therapeutic range, despite the administration of a maintenance dose of 375 mg 3 hourly, after a loading dose of 1g. It is suggested that when the oral administration of procainamide is indicated for the management of ventricular arrhythmias after myocardial infarction, a sustained release preparation should be used.