Electromyographic studies of the psoas major muscle were performed on five patients with wire electrodes implanted directly into the muscle during lumbar sympathectomy. Recordings made during various activities, approximately ten days after operation, indicated that this muscle has a regular and significant role in advancing the limb while walking and in controlling deviation of the trunk during sitting. The action of the psoas in rotation, abduction, and adduction of the hip was slight and somewhat variable, suggesting a stabilizing function.