Macrophages play important roles in host defense mechanisms. In the brain, besides microglial cells, meningeal macrophages are present. However, the pathobiological characteristics of meningeal macrophages in rats remain to be investigated. In normal meninx, immunohistochemically, macrophages reacting to CD163 (macrophage scavenger receptor) and major histocompatibility complex (MHC) class II-expressing cells (involving activated macrophages or dendritic cells) were sporadically seen without age-dependent changes. Injection of lipoplysaccharide (LPS) (5 microg; Escherichia coli) into the cerebrum increased the number of anti-CD68-positive macrophages (with greater phagocytic activity) in the meninx, with a peak at 12 h during observation period until 48 h; MHC class II-expressing cells showed a gradual increase in number from 3 h after injection; however, anti-CD163-positive macrophages did not show significant change. In in vitro studies, LPS (0, 0.02, 0.05, 0.5, 5, 50 and 100 microg/ml) was added to KMY-1 or KMY-2 cells, both of which had been established from a rat malignant meningioma. KMY-1 originally reacted to CD163, but LPS addition at 0.5 microg/ml and greater concentrations decreased the anti-CD163-positive cell number and instead increased the anti-CD68-positive cell number. LPS-treated KMY-2 increased the anti-CD163-positive cell number at 0.05 and 0.5 microg/ml. By RT-PCR methods, LPS (0, 0.5, 5, 50, and 100 microg/ml)-treated KMY-1 and KMY-2 showed an increase in mRNA of monocyte chemoattractant protein-1 (MCP-1, a chemokine), and LPS-treated KMY-2 increased mRNA of nerve growth factor (NGF, an immunological effecter). Collectively, under LPS treatment, macrophages with heterogeneous functions appear in rat meninx; rat meninx-forming cells may be involved in pathogenesis of meningeal inflammation by expressing different immunophenotypes and by producing regulatory proinflammatory factors such as MCP-1 and NGF.