The normal cardiac activation sequence requires propagation of the action potential from the subendocardial Purkinje network into the underlying Ventricular muscle cells. This process occurs at specific Junctional sites distributed over the endocardial surface of both ventricles. At these Junctional sites, action potentials can be recorded from cells that appear to be interposed between the Purkinje cells and the ventricular muscle cells. The action potential upstrokes recorded from these “transitional” cells have characteristic double phases produced by electrotonic interactions with the Purkinje cells and the ventricular muscle cells. We have shown that these Junctional regions in the canine subendocardium appear to be fixed anatomic sites with locations independent of the activation sequence of the Purkinje network. In addition, the activation delay between the Purkinje cells and the ventricular muscle cells at a Junctional site and the patterns of the action potential upstrokes of transitional cells at a Junctional site are independent of the activation sequence of the Purkinje network. We have also demonstrated that at some locations there are multiple Purkinje activation signals recorded with a surface electrode and that these multiple activation signals represent discrete groups of Purkinje cells, some of which contribute to the Junctional process while others appear to be substantially uncoupled from neighboring Purkinje cell groups and the underlying transitional cells.