The expression of MHC class I and class II molecules in the cerebral cortex of rats was investigated at daily intervals from day 3 to day 6 after fully allogeneic (DA-->LEW) and isogeneic (LEW-->LEW) kidney transplantation. MHC class II molecules were temporarily induced on the previously negative microglial cells and on the endothelia of arterioles and venules during acute rejection. On the endothelia of all brain vessels MHC class I expression was enhanced. MHC class I+ cells with microglial cell morphology were discernible within the diffusely MHC class I+ brain parenchyma. In contrast, the brain parenchyma of isograft recipients and untreated control animals did not express detectable levels of MHC molecules. In conclusion, we demonstrate that a strong immune reaction in the periphery is able to activate microglial cells in the central nervous system.