Abstract: Effects of diphenylhydantoin (DPH), PGE₂ and their combination on ouabain-induced arrhythmias were studied in anaesthetized guinea-pigs. The results showed that DPH (40 mg/kg iv), PGE₂ (0.72 μg/min) and DPH (20 mg/kg) plus PGE₂ (0.36 μg/min) decreased the incidence of ventricular fibrillation, but the protective action of co-administration of DPH with PGE₂ against ouabain-induced ventricular tachycardia was more marked than that of DPH or PGE₂ administered individually. The noradrenaline (NA) content of ventricular myocardium in animals was determined by means of fluorophotometry. The present study indicates that the antiarrhythmic action of DPH and PGE₂ may be related to inhibition of the myocardial NA release bv ouabain.