The general objectives of this project are (1) to evaluate radiation-induced changes in the immature nervous system, (2) to use ionizing radiation as a tool to study reparative and regenerative capacities of the nervous system, and (3) to study normal postnatal development of the spinal cord. The experimental animal is the rat because the development status of the rat at birth makes it possible to study and manipulate the immature nervous system ex utero. This phase of the project will concentrate on two areas. First, the proliferative activities of cells of the central canal area will be examined light microscopically and autoradiographically (3H-thymidine) after placing a small lesion in the dorsal or dosolateral region of the spinal cords. This lesion does not involve the central canal area. Second, the development of intramedullary (intraspinal) Schwann cells will be investigated with respect to age at the time of irradiation, level of spinal cord irradiated, and amount of radiation administered. The spinal cords will be examined light microscopically in order to determine the area occupied by intramedullary Schwann cells and the overall pattern of their development within the spinal cord.