We evaluated motor maps in the cerebral cortex and motor performance in cats before and after lesions of the forelimb representation in the primary motor area. After the lesion there was a reduction in the use of the affected forelimb and loss of accuracy in prehension tasks using the forelimb; some recovery occurred during the mapping study. Electrode tracts and lesion sites were located in cytoarchitectonically identified cortical areas 4gamma, 4delta, 6aalpha, 6agamma, 3a. The lesions were mainly in area 4gamma. In the lesioned hemisphere there were many points around the lesion site (in areas 4gamma and 3a) from which movements could not be evoked. In some areas distant from the lesion site (e.g. area 6agamma) the mean thresholds for evoking forelimb movements were significantly elevated. Mean thresholds for evoking hindlimb and facial movements were not different from before. In the contralateral hemisphere mean thresholds for evoking forelimb, but not hindlimb or facial movements, were significantly elevated in several sensorimotor areas (area 4gamma, 6agamma and 3a). Mean thresholds for evoking forelimb movements appeared to progressively increase during the time of study. Minimal currents required to evoke forelimb movements from the cerebral cortex increase (possibly progressively) following a lesion of the forelimb representation in the primary motor area, affecting many interconnected motor areas in the hemispheres ipsilateral and contralateral to the lesioned site. This increase in thresholds may play a role in the changes in cortical control of the affected and contralateral limbs following brain lesions and explain the increased sense of effort required to produce movements.