Thin sections of canine right and left ventricular endocardium and myocardium were studied in a tissue bath to compare conduction properties of intraventricular specialized conducting tissue [Purkinje fibers (PF)], the superficial layers of subendocardial ventricular muscle (SVM), and the deeper ventricular muscle (DVM) below this level. The study was carried out because of observations that some areas of the endocardium, which are devoid of either specialized conducting tissue or of PF-VM junctions between specialized conducting tissue and ventricular muscle, conduct relatively rapidly, favoring specific orientations of propagation. Preparations containing PF, SVM, and DVM were studied electrophysiologically and histologically. A technique of stripping limited areas of endocardium was used to expose DVM in order to determine its intrinsic calculated conduction velocity. In 12 preparations, the average calculated conduction velocity in PF was 1.62 m/sec, and the average in DVM was 0.26 m/sec. The SVM conduction velocity was intermediate between the two, averaging 0.98 m/sec when propagation was parallel to SVM fiber orientation. Conduction velocity transverse to SVM fiber orientation was not significantly different from DVM conduction velocity. Histologically, the most superficial layers of VM were oriented uniformly in the direction of rapid subendocardial conduction, in contrast to DVM fibers in which orientation varied. It is concluded that the geometric arrangement of SVM fibers may provide a means for rapid subendocardial conduction and impulse distribution at a conduction velocity intermediate between PF and DVM in areas devoid of specialized conducting tissue.