Ions injected into the circumflex branch of the left coronary artery of the intact dog produce sequential electrocardiographic changes different from those produced by exposure of the entire heart to these ions. Of all substances tested, the contour of the surface electrocardiogram is most sensitive to potassium. The electrocardiographic sequence of alterations produced by potassium so injected are characteristic and similar to those produced by subtotal and total occlusion of a coronary artery.