The utility of the antiarrhythmic drug procainamide (PA) is limited by the required dosage schedule (every three to four hours). A commercially available, slow release procainamide (P-SR) is recommended to be given every six hours. We compared procainamide capsules (P-Caps) given every four hours with P-SR given every six hours in a crossover study of 12 patients. Doses of P-Caps were chosen to produce a therapeutic result and serum drug concentrations within the therapeutic range. Doses of P-SR were adjusted until serum PA concentrations were similar to those when P-Caps were used. The dosage for P-Caps was 58 mg/kg/day +/- 24.6; P-SR dosage was 53 mg/kg/day +/- 15.18 (mean +/- SD) (P not statistically significant). The PA peak to trough variation for P-Caps was 2.2 micrograms/ml +/- 2.3 and for P-SR 1.5 micrograms/ml +/- 1.4 (P not significant). We conclude that P-SR given every six hours in the same daily dose as PA capsules given every four hours will provide similar therapeutic concentrations with no greater peak to trough variation.