A 78-year-old man with a transvenous cardioverter defibrillator system developed frequent shocks during oral procainamide therapy. Electrophysiologic evaluation demonstrated failure to terminate ventricular tachycardia with maximal energy shocks, but consistent termination by low energy discharges. Procainamide had no effect upon the defibrillation threshold. The failure of therapy with disopyramide and mexiletine to reproduce this observation suggests either a previously unreported electrophysiologic effect of, or idiosyncratic response to, procainamide.