Proliferation and differentiation of ependymal cells in the injured carp spinal cord were studied by immunohistochemistry using proliferating cell nuclear antigen (PCNA) and ³H-thymidine (³H-TdR) autoradiography. A surge of proliferation of ependymal cells occurred in the lesion with a peak around 6th day after surgical operation (complete transection) of the spinal cord. The proportion of PCNA-positive ependymal cells at 6th post-operative day (6 POD) was over 14 times that in the normal state. Electron microscopic autoradiography revealed that most of the ependymal cells incorporating ³H-TdR at 5 POD, especially those located in the caudal side of the transection site, contained numerous free ribosomes in their apical portion, but the other organellae, such as rough endoplasmic reticulum and Golgi apparatus, were poorly developed in this portion. In three weeks thereafter, the ependymal cell layer was reconstructed through the lesion, and the apical part of the ³H-TdR-labeled ependymal cells became elongated and morphologically differentiated: that is, it had free ribosomes decreased and glial filaments increased in number. Many bundles of regenerating axons were observed to course within the reconstructed ependymal cell layer. These results may suggest that proliferation, differentiation and reconstruction of the ependymal cell layer following injury of the carp spinal cord are requisite to make the permissive milieu for elongation of the regenerating axons.