Abstract: The antiarrhythmic effects of iodium-heterocycle-64 were reported in this paper. IHC-64 ⅳ 2～4 mg/kg decreased significantly the incidence of aconitineinduced arrhythmias in ratsand Ach-CaCl₂-induced artrial fibrillation(flutter)in mice, delayed the onset and shortened the duration of adrenaline-chloroform-induced arrhythmias in rabbits. The drug also increased markedly the ouabain dose to cause ventricular premature beats, ventricular fibrillation and cardiac arrest in guinea-pigs, and made BaCl₂-induced arrhythmias in rats restore immediately into sinus rhythm with the duration of more than 10 min. The incidence of ventricular tachycardia and ventricular fibrillation yielded by left coronary ligation in rats declined significantly by ⅳ IHC-64 4 mg/kg 10 min before and 2 mg/kg immediately after ligation. IHC-64 produced a bradycardia even after vagotomy, and did not block the isoprenaline-induced tachycardia and hypotention in rats.