The rodent corticospinal tract is a completely crossed projection. However, after neonatal, unilateral cortical lesion, aberrant ipsilateral corticospinal fibers develop from the unablated hemisphere. The proposed experiments are intended to investigate the functional significance of the ipsilateral tract. In collaboration with Dr. J. Ross McClung, the cytoarchitecture of the rodent spinal cord, according to the laminar nomenclature of Rexed will be described. The terminations of the normal and aberrant fibers will be determined using both the light and electron microscope. The cerebellar efferent projections coursing thru the superior cerebellar peduncle will be studied at the light microscopic level using degeneration staining techniques. Bibliographic references: Castro, A.J. 1975 Ipsilateral corticospinal projections after large cerebral lesions in neonatal rats. Exp. Neurol., 46: 1-8; Castro, A.J. 1975 Tongue usage as a measure of cerebral cortical localization in the rat. Exp. Neurol., 47: 343-352.