Intraduodenal (i.d.) administration of SIPI-644 (I) at 50, 100, and 200 mg/kg increased the threshold doses of aconitine for inducing ventricular premature beats, short ventricular tachycardia, ventricular tachycardia, and ventricular fibrillation, in rats.However, its antiarrhythmic effect did not parallel the i.d. dose of SIPI-644.The i.v. ED50 of SIPI-644 and amiodarone determined on aconitine-induced arrhythmia in rats were 12.3 and 27.8 mg/kg, resp.The i.v. therapeutic index of SIPI-644 and amiodarone were 11.7 and 5.3, resp.