Abstract: The antiarrhythmic action of lorcainide was studied on anesthesized rats and guinea pigs and compared with procainamide, verapamil, phenytoin and atropine. Intravenous bolus injection of lorcainide 3 mg/kg was shown to inhibit the cardiac arrhythmias induced by adrenaline, aconitine, BaCl₂ and ouabain, but not CaCl₂. The suppressive action of ⅳ atropine 0.1 mg/kg on adrenaline-induced rat arrhythmia was more potent than that of lorcainide. The suppressive effect of phenytoin (20 mg/kg ⅳ) on aconitine-induced arrhythmias was more significant than that of lorcainide. The effect of iv verapamil 1.25 mg/kg against BaCl₂-indueed arrhythmias was more potent than that of lorcainide; Lorcainide ⅳ 3 mg/kg inhibited ouabain-induced arrhythmias in guinea pigs. The dose required for ouabain to cause arrhythmias in lorcainide-group was more than that of verapamil, procainamide and phenytoin. Lorcainide 2.5×10^-6M also prolonged the functional refractory period of isolated cat left atria.