Two cases of congenital heart block were studied in which comprehensive serial section study of the conduction systems showed disruption of the penetrating portion of the atrioventricular (A-V) bundle. In one case, the branching portion of the A-V bundle was present together with the bundle branches. In the other case, the branching portion of the A-V bundle and the main left bundle branch were absent. The right bundle branch was interrupted after it gave off a tenuous connection to the septum; it began again, and finally terminated in the myocardium. In the latter case the electrocardiogram showed broad and notched QRS complexes. It is suggested that interruption of the A-V bundle may be related to abnormality in the mitral and tricuspid extensions of the central fibrous body or of the pars membranacea. This malformation of the conduction system is not related to any one type of congenital anomaly.