This is a quantitative study of changes in distribution and density of terminals of the corticospinal tract in the cervical spinal cord and dorsal column nuclei (DCN) in cats with left cerebral hemispherectomy performed neonatally or in adulthood. Kittens received hemispherectomy at a mean of 12.1 postnatal days and were compared, as adults, to adult-lesioned cats of similar survival time. All animals, including controls, received injections of [3H]leucine-proline and were sacrificed 5 days later. Injection sites and terminal fields were reconstructed from autoradiography-processed tissue. The label filled comparable extents of areas 4 gamma and 3a of the right cerebral cortex. Coronal sections from upper and lower cervical cord levels, and from the brainstem (cuneate and gracile nuclei) were studied. Computer-image processing procedures were used to count labeled particles from multiple sites of the dorsal horn and DCN, bilaterally. In the spinal cord of intact and adult-hemispherectomized cats, most terminals were found in lamina VI, and adjacent laminae V and VII contralateral to the injection side. The major finding was that neonatal-lesioned cats showed a significant increase in axon terminals in areas ipsilateral to the injection. The topography of distribution of the novel terminals was similar to that in the contralateral side and the originating fibers appeared to have crossed the midline from that side. A similar reorganization occurred in the gracile nucleus where, in intact and adult-lesioned cats, the cortical terminals also predominated in the side contralateral to the injection. In contrast, neonatal-lesioned animals showed a significant increase in terminal density ipsilateral to the cortical injection. These findings are discussed as an alternative mechanism for postlesion remodeling of the corticospinal tract in animals with the pyramidal crossing completed at the time of birth.