The effects of altering the conductivities of the boundary media at the endocardial and epicardial surfaces of the ventricle on accession potentials in the ventricular wall and at its surfaces were studied in dogs. The amplitude of R at all points in the wall was decreased by any measure which increased the conductivity of the medium surrounding the heart or which decreased the conductivity of the medium in the ventricular cavity. The progression of R in the ventricular wall observed experimentally results, at least in part, from the differing conductivities of intracavitary blood, myocardium, and the medium surrounding the heart.