Experiments with recording the time of intraventricular reentry of the canine heart by a special protocol of stimulation showed that the first-line antiarrhythmic agents having various kinetics, ethacizine and lidocaine, compete for their binding to the Na+ channels of cardiac fibers. As a result of this competition, additivity was not found in the effects of ethacizine, 1.5 mg/kg, and lidocaine, 12 mg/kg, on the rate of intraventricular conduction at heart rate under 180/min. At higher cardiac rhythm values, the total effects of the two agents on the conduction were observed. The experimental evidence confirm the results of mathematical modelling of the combined effects of ethacizine and lidocaine on the intraventricular conduction velocity, which were calculated on the basis of kinetic constants for binding and dissociation of the agents to Na+ channels. The findings show that when the two first-line antiarrhythmics having a substantially different kinetics were used in combination, their antiarrhythmic effect may be enhanced without their total effects on the rate of normal intraventricular conduction velocity reentry.