The use of radiation to improve the efficacy of chemotherapy on malignant brain tumors is also known to cause side effects on vascular endothelial cells and astrocytes in normal parts of the brain. We investigated the effects of lipopolysaccharide (LPS) on the functional and structural properties of blood-brain barrier (BBB) and the activity of astrocytes during whole-brain irradiation in rats. The permeability of the BBB to Evans blue (EB) dye significantly increased in the cerebral cortex, diencephalon and cerebellum regions of rats exposed to irradiation (P<0.01). In contrast, the BBB permeability in irradiated rats was significantly reduced by LPS (P<0.05). Tumor necrosis factor-alpha (TNF-alpha) levels were increased following LPS, irradiation and irradiation plus LPS (P<0.05, P<0.01). Irradiated brain vessels showed a considerable loss of staining intensity of tight junction proteins Zonula occludens-1 (ZO-1) and occludin. Staining for Zonula occludens-1 and occludin was intensive in animals treated with LPS and irradiation plus LPS. Glial fibrillary acidic protein (GFAP) immunoreactivity was seen in very few astrocytes of irradiated brains. However, this staining showed an increased positive intensity in the brain sections of LPS-treated as well as of irradiation plus LPS-treated animals. These results indicate that LPS reduces the passage of exogenous vascular tracer EB-binding albumin into the brain, at least partly, by increasing the expression of tight junction proteins and GFAP, following the irradiation. We suggest that irradiation may affect paracellular permeability through disruption of tight junction proteins, Zonula occludens-1 and occludin, and LPS could provide beneficial effects on the BBB integrity and the astrocytes against irradiation damage.