Premature systoles were elicited at many points in the dog heart and the resultant pattern of excitation plotted in detail with a multipolar electrode and a 16-channel oscilloscope. In premature systoles, most of the mural endocardium is excited by a wave travelling at about 1 meter per second. These results indicate that in a normally originating beat the mural endocardium is excited by branched Purkinje tissue, the elements of which conduct at about 1 meter per second. The transmural velocity of the wave of depolarization in premature systoles is about 0.3 meter per second. No evidence was found for intramural penetration of Purkinje fibers. Intramural potentials recorded on stimulation give some insight into normal intramural potentials.