The aim of this study was to investigate the role of microglia in radiation-induced astrocyte gliosis. We found that a single dose of 15 Gy radiation to a whole rat brain increased immunostaining of glial fibrillary acidic protein in astrocytes 6 h later, and even more so 24 h later, indicating the initiation of gliosis. While irradiation of cultured rat astrocytes had little effect, irradiation of microglia–astrocyte mixed-cultures displayed altered astrocyte phenotype into more processed, which is another characteristic of gliosis. Experiments using microglia-conditioned media indicated this astrocyte change was due to factors released from irradiated microglia. Irradiation of cultured mouse microglial cells induced a dose-dependent increase in mRNA levels for cyclooxygenase-2 (COX-2), interleukin (IL)-1β, IL-6, IL-18, tumor necrosis factor-α and interferon-γ-inducible protein-10, which are usually associated with microglia activation. Consistent with these findings, irradiation of microglia activated NF-κB, a transcription factor that regulates microglial activation. Addition of prostaglandin E₂ (PGE₂: a metabolic product of the COX-2 enzyme) to primary cultured rat astrocytes resulted in phenotypic changes similar to those observed in mixed-culture experiments. Therefore, it appears that PGE₂ released from irradiated microglia is a key mediator of irradiation-induced gliosis or astrocyte phenotype change. These data suggest that radiation-induced microglial activation and resultant production of PGE₂ seems to be associated with an underlying cause of inflammatory complications associated with radiation therapy for malignant gliomas.