Prifuroline is an antiarrhythmic agent. Prifuroline dose-dependently antagonizes the arrhythmogenic action of aconitine in rats, when administered either intravenously (5, 10 or 20 mg/kg) or intraduodenally (10, 20 or 50 mg/kg). Prifuroline also diminishes ventricular susceptibility to electrical stimulation in open-chest rats. The arrhythmias induced in dogs by coronary artery ligation are markedly antagonized by Prifuroline after doses of 5 and 10 mg/kg i.v. or 30 mg/kg intraduodenally. The duration of its antiarrhythmic activity in this model of arrhythmias in conscious dogs is much longer after intraduodenal than after i.v. administration.