These experiments examined the ability of the adult motor cortex to reorganize its relationship with somatic musculature following nerve lesions. Cortical motor output organization was assessed by mapping the areal extent of movements evoked by intracortical electrical stimulation in anesthetized rats. Output patterns of the motor cortex of normal rats were compared with those of adult rats that had received either a forelimb amputation or a facial motor nerve transection 1 week to 4 months earlier. In both experimental conditions the extent of some representations increased. Stimulation thresholds required to elicit movements in expanded representations were at or below normal levels. After forelimb amputation, the area from which shoulder movements could be evoked at low thresholds enlarged. Sectioning of the branches of the facial nerve that innervate the vibrissa musculature enlarged the motor cortex forelimb and eye/eyelid output areas; these enlargements appeared to occupy the former vibrissa area. These results indicate that the amount of cortex controlling a group of muscles and the strength of the relationship between a cortical locus with its target muscles is modified by nerve lesions in adult mammals. They also show that motor nerve lesions are sufficient to produce this change and that the changes can appear as early as 7 days following a peripheral nerve injury.