Radiation-induced apoptosis has been reported in thymic, lymphoid, haematopoietic cells and intestinal epithelium but is infrequently documented in other adult mammalian cell types. In this study, we examined the time course of radiation-induced apoptosis in the adult cervical rat spinal cord following a single dose of 8 or 22 Gy. Apoptosis was assessed by morphological criteria under light and electron microscopy, and immunohistochemically in-situ using Apoptag to detect 3' -OH ends of DNA fragments. Little evidence of apoptosis (0.3 +/- 0.1 apoptotic nuclei per spinal cord section) was observed in control un-irradiated spinal cord. A significant increase in the number of apoptotic cells per spinal cord section was seen at 4 h after 8 (13.6 +/- 1.3) or 22 Gy (22.0 +/- 2.7). The number of apoptotic nuclei reached a peak at 8 h (44.7 +/- 3.7 after 8 Gy, 49.5 +/- 4.3 after 22 Gy), and returned to the baseline level by 24 h (2.4 +/- 0.7 after 8 Gy, 3.3 +/- 0.7 after 22 Gy). A dose of 22 Gy induced significantly more apoptoses than 8 Gy at 4, 6, 10 and 12 h (P < or = 0.033), but not at 8 h. More apoptotic nuclei were observed in white matter (64-92%) than gray matter (8-36%). All the apoptotic cells were observed in glial cells, and there was no evidence of radiation-induced apoptosis in the vascular endothelial cells or neurons. The morphological features of the apoptotic cells under electron microscopy and the absence of GFAP staining suggested that they were oligodendrocytes. We conclude that radiation induces apoptosis in the adult rat spinal cord, and that the development of apoptosis follows a specific time course.