Using glial fibrillary acidic protein (GFAP) immunohistochemistry, and electron microscopy, we examined changes in the features of astrocytes in the dentate gyrus of the hippocampal formation after high-dose chronic toluene inhalation (2000 ppm, 4 h/day) for 1 month. In toluene-treated rats, the numbers of GFAP-immunoreactive (IR) astrocytes were not changed, whereas the area and intensity of GFAP-IR processes were increased markedly in the dentate gyrus. In addition, dense, thick and highly-stained GFAP-IR processes appeared entering into the granular cell layer, compared with those of controls. At the electron microscopic level, numerous astrocytic processes with high electron density intervened between the granular cells. These results suggest that high-dose of chronic toluene exposure induces morphological changes in astrocytes in the dentate gyrus of the hippocampal formation.