After injuring the hypoglossal nerve the neuroglia cell reaction was studied in the region of the neuronal perikarya in the hypoglossal nucleus of both rabbit and mouse. Radioautography was used to determine the rate of cell proliferation; it was demonstrated that ³H-thymidine could serve as a precursor for DNA synthesis for more than 2 hr after intracisternal injection of it in the rabbit. The labeled neuroglia cells in the regenerating hypoglossal nucleus of the rabbit were predominately microglial from 2 hr up to 3 weeks after isotope injection. The life-span for a large part of the labeled cells was at least 3 weeks. After nerve section the neuroglial proliferative response was more marked than after a single crushing, whereas no additional proliferative reaction could be observed in the rabbit after a repeated crushing either 14 or 49 days after the initial injury. In contrast to the almost complete survival of nerve cells and reversibility of the astrocytic hypertrophy after crushing the hypoglossal nerve, nerve section gave a 10% reduction in number of nerve cells and a persistent astrocytic hypertrophy for at least 3 months. A marked neuroglial proliferation was found on the 2nd and 3rd day after crushing the mouse hypoglossal nerve. No labeled cells were demonstrated in the vascular walls, as was the case in the rabbit. No evidence for a hematogenous source of these proliferating cells could be obtained in mice given closely repeated injections of ³H-thymidine to label leucocytes.