The electrical manifestations produced in the cardiac muscle by ischemia and toxic substances have been studied. In the damaged cardiac muscle the resting membrane potentials may be zero, subnormal, or supernormal. Damage producing block of the excitation wave may be observed without altering the resting membrane potentials. Accordingly, monophasic action potentials may be observed with or without currents of injury, and even with currents of injury, against the rule. In recording monophasic waves the only essential condition is block of the excitation wave. The presence or absence of injury currents is immaterial. In the damaged cardiac muscle the excitation wave is blocked. The intensity of block varies with the degree of damage. When block is severe, the excitation wave does not progress in the damaged tissue and monophasic action potentials are recorded. When block is less marked, the depolarization and repolarization processes are progressing slowly, and, thus, QRS-T complexes abnormal in size and width are registered. In the less damaged zone only the recovery process is delayed and primary T-wave changes recorded. © 1957.