Data from 5,280 fibrillation-defibrillation episodes in medium-size dogs, size scaled to a hypothetical 100-kg dog, and from 3,240 episodes in 100-kg calves, are used to derive families of curves showing percentage of success of unidirectional square-wave pulses in achieving defibrillation versus current amplitudes for energy levels in the range of 200 through 2,000 J. From the 3,240 episodes in the calves and 3,240 additional episodes in dogs, the incidence of post-defibrillation complete A-V block was found. In calves, the optimal 800-J curve peaks sharply at about the 93 percent success level for a current amplitude of 70 A. For the hypothetical dog, the 800-J curve has a broad peak at substantially the 100 percent success level for currents of 40 through 120 A. In general, the incidence of complete A-V block is very low in the dog and very high in the calf.