Multipolar recording techniques have been used to study excitation of the canine interventricular septum. Activity commences in the mid left septal endocardium and slightly later near the right anterior papillary muscle. From these sites depolarization spreads over the endocardial surfaces, probably via the Purkinje tissue, at 1 m./sec. and through the septal thickness at one-third this speed. Near the upper septum, bilaterally and on the posterior right there is a lack of the rapid endocardial conduction. Septal contribution to the QRS is considered.