From tracings of the movements of the apex and base of the ventricle of the dog, frog and turtle, it is deduced that the major pumping action of the heart is a reciprocating movement of the A-V septum which enables the auricles to fill while the ventricles are emptying and vice versa. Volume records of the auricle and ventricle, taken simultaneously or allowed to integrate graphically substantiate this reciprocal volume relationship and also show that the volume of the whole heart changes but slightly during the cardiac cycle. Additional corroborative evidence was furnished by studying movements of the A-V septum in a normal healthy dog whose A-V groove had been visualized roentgenographically by metal particles inserted during a previous operation.