AbstractUnilateral spinal lesions were made in six Macaques; in two, the bulbar pyramid was sectioned contralaterally. In four Macaques with chronic pyramidal section, contralateral spinal lesions were made. The effect of each lesion separately and of the combined lesions upon the locomotor status of the animals was evaluated behaviorally, by electromyography and the lesions studied histologically after two years' survival.Spinal lesions provoked changes in resistance to passive stretch and tendon reflexes primarily, and changes in movement secondarily: when hypertonus appeared, movements in opposing muscles were restricted, whereas movements opposing muscles were restricted, whereas movements opposed by muscles with decreased tone were hypermetric. Spinal lesions greatly altered the paresis provoked by pyramidotomy: small ventrolateral lesions restored “contact” placing, improved stepping and hopping and made the hypotonic (“pyramidal”) limb hypertonic. Execution of movements was poor, however, poorest in an animal with combined pyramidal and ventral funiculus lesion. The importance of extrapyramidal tracts in return of function following pyramidotomy was demonstrated in relation (a) to the size of the spinal lesion and (b) to its cross‐sectional location.Gross sequential movement of muscles normally fixating the limb appears when the pyramid is cut. These movements are greatly impaired when spinal lesions are added, and the usual grouping of synergists is disrupted. Fixation of the limb represents the integration of extrapyramidally activated, pyramidally inhibited synergistic contractions upon which discrete usage of distal muscl