Mitotic-linked death is generally regarded as the mode of radiation-induced cell death, particularly in late-responding normal tissues, such as those found in the central nervous system. We have recently reported evidence for radiation-induced apoptosis in the central nervous system using the adult rat spinal cord model. Glial cells, but not neurons or vascular endothelial cells, appeared to undergo apoptosis within 24 h of irradiation. To further characterize the apoptotic process and the type of glial cells involved, a 2-cm segment of the adult rat cervical spinal cord was irradiated with single doses of 1-30 Gy and processed for detailed histological examination at 0, 4, 8, 12, 16, and 24 h after irradiation. Apoptosis was assessed using standard morphological features under the light and electron microscopes and an in situ end labeling assay. A dose response for radiation-induced apoptosis was observed over a dose range of 1-30 Gy, with the peak response at 8 h after irradiation. At 8 h after a 22-Gy irradiation, 96.1% of the apoptotic cells showed positive immunohistochemical staining with Leu-7, a specific marker for oligodendrocytes; only 4.4% of apoptotic cells were positive for Ricinus communis agglutinin-1 (a marker for microglia), and none were positive for glial fibrillary acidic protein (a marker for astrocytes). A significant decrease in the total glial cell density was observed at 24 h after irradiation with 22 (11%) or 30 Gy (14%) but not with 8 Gy. This was due primarily to a decrease in the oligodendroglial density (24%, 22 Gy, P < 0.001; 19%, 30 Gy, P = 0.001), because no decrease in the astroglial population was observed. The duration of apoptosis was estimated to be approximately 1 h. We conclude that there is a depletion of the oligodendroglial population in the adult rat spinal cord within 24 h after irradiation and that the mode of this radiation-induced cell death is apoptosis.