Diphenylhydantoin (DPH) and potassium significantly prevent ouabain intoxication without preventing the inotropic effects of ouabain in the guinea-pig isolated heart.2. The antiarrhythmic effect of DPH and K on ouabain-induced toxicity appears to be related to their ability to reduce ouabain accumulation by the myocardium and thereby prevent the intracellular Na and K changes which lead to the arrhythmic state.