In canine Purkinje fiber-papillary muscle preparations it was shown directly that electrotonic spread can take place across the junctions between Purkinje fibers and ordinary muscle fibers (P-M junctions). The P-M delay recorded during orthodromic propagation varied considerably within any given preparation. Moderate increases in the external concentration of K + (up to 6 ITIM) consistently decreased the P-M delay; when the concentration of external K + was increased to 8 HIM the P-M delay increased. With larger concentrations of K + (10 to 11 mM), total conduction block from the terminal Purkinje fibers to the muscle occurred; under the same conditions antidromic propagation from muscle to Purkinje fiber was still possible. The results can be explained satisfactorily by assuming that propagation across the P-M junction is electrical and that the geometry of the functional syncytium changes progressively from a cable-like system at the level of the terminal Purkinje fibers to a two-or threedimensional irregular syncytium at the bulk of the myocardial mass.