PURPOSE: Traumatic brain injury is a multifaceted injury that involves direct mechanical damage, intraparenchymal and subarachnoid hemorrhage, breakdown of the blood- brain barrier, excitotoxicity, and ischemia. Despite the dozens of previous investigations, the information about its pathogenic mechanism is still limited. The aim of this study was to reveal the appearance of antigen presenting cells in the cerebral cortex of rats after cauterization. METHODS: A total of 18 male Sprague-Dawley rats weighing 300 g and 2 months old on the average were used throughout the experiment. The frontal bones were exposed by elevating the skin and craniectomies were performed adjacent to the central suture, midway between lambda and bregma. Cauterizing injury was then created by battery-operated small vessel cauterizers to the left frontal cortex. The rats were sacrificed on the 1st, 4th, 7th and 14th days after the surgery(n=3, each time), and three rats were sacrificed as normal controls. Serial brain cryosections were made by cryostat. For immunohistochemistry, brain tissue sections were allowed to react with mouse anti-rat MHC class II antibody(1:500) and mouse anti-rat ED2 antibody(1:200). Also, brain tissues were routinely stained by H-E, and then microscopic observation and cell counts were performed. RESULTS: 1) MHC class II positive dendritic cells were absent in normal cerebral cortex parenchyme, but were found 28 times more in number in injured rats on the 7th day after cauterization. 2) ED2 positive macrophages were absent in normal cerebral cortex parenchyme, and were found 16 times more in number in injured rats on the 7th day after cauterization. 3) The number of MHC class II positive dendritic cells were smaller in number than that of ED2 positive macrophages 6 hours and 1st day later after cauterization, but it was higher in number on the 4th, 7th and 14th days. 4) The number of MHC class II positive dendritic cells were higher in number than that of ED2 positive macrophages around blood vessels and peripheral regions in the injured brain. 5) MHC class II positive dendritic cells were usually aggregated. CONCLUSION: It can be suggested that the increase in number of two kinds of antigen- presenting cells affect cell-mediated immune responses and phagocytosis.