In an experiment on defibrillation of the myocardium in dogs, a distinct advantage of large electrodes (7.3 cm.) over small (5 cm.) was demonstrated. A survey of defibrillation equipment in use in 26 hospitals revealed that in 13 hospitals only electrodes 5 cm. or less in diameter were available for human use. During defibrillation, marked heating of the myocardium to over 65° C. occurred when voltages exceeding 170 were used. The use of 100 to 170 volts was safe with shocks of alternating current of 0.15- to 0.30-sec. duration repeated up to 10 times in 10 sec. Single shocks of 0.15 sec. were safe up to 380 volts if large paddles (8 to 12 cm.) were used. To avoid thermal injury and to effect prompt defibrillation, a heart-shaped electrode measuring 8 by 12 cm. is recommended when intrathoracic defibrillation is practiced.