The temporospatial relationship between microglial and astrocytic reactions and delayed thalamic cell death was examined 1-7 days following a traumatic cold lesion of the rat sensorimotor cortex using immunocytochemistry in combination with terminal deoxynucleotidyltransferase-mediated biotinylated dUTP nick end labeling (TUNEL) of nuclear DNA fragmentation. No or only occasional TUNEL-positive cells were found in the thalamic relay nuclei up to 3 days after trauma. After 7 days, on the other hand, a considerable number of TUNEL-positive cells were seen in the ventrobasal, the ventrolateral and posterior thalamic nuclei. Already 3 days after trauma, i.e., before cell injury was detectable, many protoplasmic astrocytes, which were reactive for glial fibrillary acidic protein, and ramified microglia, which were positive for complement receptor type 3b (CR3b) but negative for major histocompatibility complex (MHC) class II antigen, were noticed in the thalamus. The number of labeled astro- and microglia further increased after 7 days, when DNA fragmentation became evident. At this time, the morphology of microglia shifted towards bushy and rod-like cells, and microglia became also reactive for MHC class II antigen. Clusters of CR3b- and MHC class II-positive microglia were found in the ventrobasal thalamus. The present findings demonstrate that trauma-induced microglial and astrocytic reactions appear in the thalamus prior the onset of cell damage.