Abstract: Iv NICO 100mg/kg significantly counteracted the arrhythmias induced by aconitine 20 μg/kg or BaCl₂ 4 mg/kg in rats, and that by chloroformadrenaline in rabbits. NICO was found to decrease the incidence of ventricular fibrillation in rats, increase the tolerant dose of ouabain to produce ventricular permature beats, ventricular tachycardia, ventricular fibrillation and cardiac arrest in guinea pigs. Iv NICO 50 mg/kg, significantly prevented the arrhythmias induced by ligation of the left coronary in rats. In mice, NICO 50～100 mg/kg ip decreased the incidence of ventricular fibrillation by chloroform and artrial fibrillation (flutter) by ACh-CaCl₂. NICO produced bradycardia and antagonized the positive chronotropic effect of isoprenaline induced in guinea pigs.