A method of producing focal ventricular block is described. The sequence of the electrocardiographic variations is ascribed to changes in the velocity and direction of the excitatory process in the ventricular wall. The epicardial electrocardiograms resemble those considered indicative of ventricular hypertrophy or of "incomplete" or of "complete" bundle branch block. When the focal block is pronounced a positive deflection appears in the cavitary tracing. The ventricular blocks can be subdivided into "conduction blocks" and "fiber blocks," the former produced by the delay of the stimulus in the specialized conduction system and the latter produced by the delay of the excitatory process in the ordinary heart muscle.