In studying the effect of electric shock upon rats (1, 2, 3) it became evident that the injury was in the main confined to the portion of the body that was traversed by the current. The, present paper is directed specifically to a study of the effect upon the heart since this organ is believed to be especially liable to injury in electric shock. The purpose of the present experiments was two-fold; to ascertain the proportionate value of the total current which actually flows through the heart when contact at various points on the body is made with the circuit and to determine the minimum current necessary to establish ventricular fibrillation. Dogs, completely anesthetized with morphia and ether, were used as experimental animals because the ventricles of the dog’s heart are readily thrown into a permanent state of fibrillation by the application of relatively weak currents as is assumed to be the case in man. The source of the current was a 60 cycle, alternating current circuit. Ring or through type current transformers were employed to measure the current traversing the heart, the heart itself forming the primary winding of the transformer. The iron cores of these transformers were ring shaped and of the highest quality magnetic material.’ They were approximately square, having a cross-sectional area of 0.01 square inch (0.07 sq. cm.). Each ring was uniformly wound with approximately 2000 turns of no. 36 Brown and Sharp enamel insulated copper wire which served as the secondary winding on the transformer. The rings were of varying inside diameters, ranging from Q inch up to 3# inches, so that they could be slipped over hearts of different sizes. The current flowing through the heart induces an electro-motive force in the secondary winding of the transformer encircling it. The terminals of this secondary winding were connected across a potentiometer type rheostat, and the desired portion of the voltage drop in this rheostat was fed into an amplifier whose