The effects of EDTA on the transmembrane potentials and mechanical events of the ventricular muscle of rabbits were studied under different ionic conditions. EDTA (5 x 10-3[image]) caused a relaxation and complete disappearance of the contraction in spite of the appearance of slight depolarization of the membrane, and marked prolongation in duration of the action potential. An increase of the latent period and a rise in threshold were also noticed before automaticity of the muscle occurred. After reintroducing of normal Tyrode solution, small oscillation appeared on the plateau and developed to a full re-polarization of the membrane, when exceeded a critical value. The potential, soon returning up to the plateau from the resting potential level, shifted between these two stable states spontaneously and repeatedly. When EDTA was added to and NaCl was depleted from the solution, the level of plateau lowered and the duration rather shortened despite the action of EDTA. The lowering of spike height was continuous but the plateau potential stayed for a while at a level about[long dash]17mV and later diminished. Under these conditions the action potential appeared often divided into 2 components, spike and plateau. When EDTA was added and KC1 was depleted from the solution, the prolongation of plateau was more remarkable than when EDTA alone applied. But the prolongation was canceled and rather a shortening of the plateau appeared when Na+ was lack in the solution. Oscillation of increasing amplitude was also observed toward the end of plateau when KCl-lack EDTA solution was replaced by normal one. EDTA-added NaCl-excess Tyrode caused first an increase in height of both spike and plateau and then a decrease probably due to the hypertonicity of solution. EDTA-added KCl-excess Tyrode, on the other hand, produced a decrease of the spike and later the plateau. From these results it is assumed that the transmembrane action potential of the cardiac muscle has at least 2 components, spike and plateau, each of which relates principally but differently to Na + ions, The plateau potential was further estimated to have some relation with Ca++-chelating action on the membrane of the contractile elements of the muscle during the tension development.