The kinetics of tocainide, a new antiarrhythmic, and two of its metabolites, lactoxylidide (LX) and tocainide carbamoyl glucuronide (TG), were examined in patients given tocainide for 7 days following an acute myocardial infarction. In these patients kinetics were much the same as those reported for volunteers and for patients treated with tocainide for chronic extra ventricular beats. Mean half-life for tocainide, LX, and TG were 13.6, 29.1 and 13 hr. At steady state mean metabolite to tocainide ratios in serum were 0.48 for LX and 0.28 for TG. Using animal models, we examined the relative antiarrhythmic, direct cardiac, and central nervous system (CNS) effects of tocainide and LX. LX, the deaminated alcohol metabolite, had no antiarrhythmic, direct cardiac, or CNS toxic effects. These data suggest that tocainide, unlike many antiarrhythmic drugs, has no active metabolites.