Electrophysiologic mapping experiments on the excitation of the A-V node of isolated rabbit heart have been performed with intracellular microelectrodes. An area of tissue showing peculiar physiologic characteristics was shown to coincide, at least in part, with the histologic A-V node and His bundle. Three functional regions have been determined in the physiologic mapping of the A-V node. The middle layer (N) shows signs of decremental conduction and is the site of the slowest propagation velocity in the A-V node. The other layers are transitional regions between the N layer and atrium (AN) or the His bundle (NH). The AN layer shows a progressively slower propagation velocity as the N region is approached. The same holds for the NH region. A precise correlation between these findings and the 3 existent histologic layers remains to be determined. The existence of an area of peculiar tissue just above the A-V valves and encircling the A-V orifice has been confirmed. This area is continuous with the AN layer of the functional A-V node and has been named the atrioventricular ring (AVR) on the assumption that it is a remnant of the embryonic A-V ring. Wenckebach cycles and A-V block of nodal origin are tentatively explained as resulting from increased decrement in the middle nodal layer (N).