Both isomers of propranolol are capable of preventing adrenaline-induced cardiac arrhythmias in cats anaesthetized with halothane, but the mean dose of (-)-propranolol is 0.09 mg/kg whereas that of (+)-propranolol is 4.2 mg/kg.Both isomers of propranolol are capable of reversing ventricular tachycardia caused by ouabain in anaesthetized cats and dogs. The dose of (-)-propranolol is significantly smaller than that of (+)-propranolol ((R)-Propranolol) in both species.(R)-Propranolol hydrochloride (10 mg/kg) has no antagonistic activity in rats, the cardiac responses to isoprenaline of rat can be blocked by probably much less than 1/100th of that of the (-) isomer in this preparation.