SINCE the introduction of electric shock in the treatment of certain mental disorders, numerous investigators have attempted to determine whether this form of therapy produces histologic changes in the central nervous system. It has not been shown conclusively that changes do occur. The problem was restudied by subjecting monkeys to treatment similar to that employed in human electric convulsion therapy. After shocks of various duration, the animals were killed and the central nervous system was prepared for histologic examination by improved and rigidly controlled technical procedures that precluded postmortem cellular deterioration. MATERIALS AND METHODS Five male monkeys (Macaca mulatta), weighing 8 to 10 pounds (3.6 to 4.5 Kg.), were treated; an additional monkey served as a control. All were kept together in a large cage and were fed an adequate diet of fresh fruits and vegetables and the standard prepared laboratory chow. Throughout the experiment, the animals appeared healthy, showed