In order to assess the role of circulating blood in early microglial activation after traumatic brain injury (TBI), controlled cortical impact injury was applied to adult rat brain slices (400 microm in thickness) and the microglial response was examined. The complement receptor (CR3) expression and morphological transformation of the microglia were evaluated by OX42 immunohistochemistry. At 5 min following injury, activated microglia with intense CR3 expression appeared throughout the hemisphere on the injured side. In contrast, the morphology and CR3 expression of the microglia on the contralateral side were indistinguishable from those of the resident ramified microglia seen in normal brains. At 30 min following injury, microglial activation was more pronounced on the injured side, while the microglia on the contralateral side still retained a ramified morphology. These results are consistent with our previous observations made in in vivo experiments, which indicate that, as the brain slice paradigm excludes variables arising from the circulating blood, the rapid and widespread microglial activation observed following TBI can not be attributed exclusively to the infiltration of blood-borne macrophages or molecules. Rather this activation is most likely caused by intrinsic mechanisms within the brain tissue, such as traumatic depolarization.