T HAS been long accepted that after complete occlusion of a coronary artery the resting (diastolic) polarization of the myocardial fibers in the area supplied by the occluded vessel is reduced. The degree of diastolic polarization varies in the different zones of the infarcted area and determines whether a zone is completely unexcitable (dead zone), or is activated with a delay (injury zone) or presents only alterations in its recovery or repolarization (ischemic zone).'. 5 The dead zone is identified electrocardiographically by abnormalities of the QRS complex (Q waves or QS complexes), the injury zone by displacerrlents of the RS-T segment and the ischemic zone by alterations of the T wave (inversion of the T wave or equivalent changes). All electrical differences of membrane potential in the cardiac fiber have their source in the uneven distribution of inorganic ions on the two sides of the membranes-n Studies with isotopically-labeled tracer substances reveal that maintenance of the ionic gradients by tissues requires one or more" pumping" mechanisms which, in the final analysis, must depend upon energy derived from the cellular metabolism 12-14 The concentration of K+ inside the fiber (K;+) is about 30 times higher than that outside the fiber (Ko+). If one accepts the figures given by Robertson and Dunilmets for the heart muscle of the cat, by applying the Nernst formulat to K,+ and Ko+ concentrations, the calculated potential difference across the resting membrane is approximately 92 inv.(the outside is positive with respect to the inside), a value within the range (70 to 100 mv.) of the resting membrane potentials measured directly with microelectrodes in mammalian cardiac fibers.'," The concentrations of other ions (Na, Cl-, Ca++ and Mg-+) in the extracellular fluid exert little or no effect on the magnitude of the transrnembrane resting potential."