The response of ependyma in the fourth ventricle and central canal to injury was investigated in young Sprague-Dawley rats. Animals received either a small unilateral incision in the lateral funiculus of the thoracolumbar (TL) cord or a puncture wound in the dorsolateral medulla. Control and lesioned rats were killed from 2 to 21 days after operation and the tissue encompassing the lesions was processed for light microscopy and transmission electron microscopy. Randomly selected lesioned and control rats received colchicine (0.2 mg/100 g s.c.) 6 h prior to death at postoperative days 1-4. In controls, the central canal of the TL cord was lined by a single layer of uniformly arranged columnar ependymal cells with large basally located nuclei and apically displaced organelles. Prominent membrane fusions and condensations of filaments were consistent features of the apical region of these cells. Their basal poles abutted directly on the subjacent neuropil and occasionally gave rise to radially directed processes. In rats with TL lesions, the lumen of the central canal was collapsed and lined by an irregularly contoured, multilayered appearing ependyma. Ependymal cells were often radially elongated and possessed filament-filled basal processes. Mitotic rates among ependymal cells of lesioned rats reached a maximum of 3.34% on day 2 compared to 0.21 and 0.28% in intact and sham-operated controls, respectively. Mitotic activity declined progressively thereafter and approached control values by day 4. A similar but less pronounced pattern of proliferative activity was seen distal to the cord lesion, however, little activity was observed within the lining of the fourth ventricle of animals with comparable lesions in the medulla.(ABSTRACT TRUNCATED AT 250 WORDS)