Compared to procainamide in an animal arrhythmic model, the antiarrhythmic potency of the N-acetylated metabolite of procainamide (NAPA) was 92% with respect to dose and 70% with respect to plasma level. The antiarrhythmic effects of combinations of the drugs were additive. Measurements of procainamide and NAPA plasma levels needed to suppress ventricular extrasystoles suggested that both compounds are nearly equipotent in patients as well. The average plasma level required for arrhythmia control in these patients was equivalent to 5.1 mcg/ml procainamide. Since patients on long-term procainamide therapy have plasma concentrations of NAPA that are usually comparable to, and occasionally greater than, their procainamide levels, dose regiments based on procainamide levels alone need revision to include consideration of the levels of this metabolite.