The effects of bound ouabain on the electrogram of the isolated frog heart are antagonized by epinephrine and potentiated by calcium. Ouabainized hearts also develop increased ventricular automatism (paroxysmal tachycardia, flutter and fibrillation) in the presence of large concentrations of calcium but not of epinephrine. The actions of both epinephrine and calcium are readily reversed by washing the tissue with normal Ringer's solution.