The histogenesis of the atrioventricular (A-V) node and bundle was studied in 11- and 12- day-old mouse embryos. In younger embryos (9 and 10 days postcoitum), the inner cell layer of the dorsal wall of the A-V canal forms a specialized interconnecting tissue between the atrial and ventricular muscles. The A-V node originates from the cells at the distal end of this specialized tissue and proliferates into the loose mesenchyme of the dorsal A-V cushion. The proliferation takes place proximal to the developing interventricular (IV) septum with which the specialized inner layer of the A-V canal is interconnected. At the same time a PAS-positive cell group differentiates along the ridge of the developing IV septum and extends from the primitive node to the middle portion of the rim of the IV foramen, where it divides into left and right branches. In 11- and 12-day embryos, the A-V node and His bundle primordia interconnect with the ventricular trabeculae and are in contact with the myocardium of the IV septum. In the 12-day embryos, blood capillaries and connective tissue cells begin to isolate the primordium of the conducting system from the force producing myocardium. Incomplete isolation of the conducting and contracting muscle cells may result in the persistence of aberrant A-V connecting fibers.