The electrical threshold of ventricular fibrillation induced by premature single impulses and the ectopic activity and also an abnormal conduction during vagus inhibition of sinus node were estimated 24 hours after the administration of isoproterenol (10 mg/kg, s/c, on time) in rats. In addition the cardiac contractile function of the left ventricle was studied. The study was performed on male Wistar rats, 250-300 body weight, under nembutal anesthesia. Isoproterenol had no effect on the contractile function in the rest and during maximal isometric load, induced by coarctation of ascending aorta. But in the treated animals the threshold of fibrillation fell more than 2-fold and the vagal bradycardia was more 2-fold then in the untreated animals. The AB-block, idioventricular rhythm and extrasystoles appeared during vagal bradycardia in treated animals, while in the untreated ones there were no any disturbances. The preliminary administration of the antioxidant ionol (BMT, 30 mg/kg, per os, in sun oil) prevented the enumerated shifts.