The histopathology of the brains of rats killed was studied, fo11owing various doses of 20 to 30 Gy X rays with or without cisplatin (cis-diamminedichloroplatinum II) treatment. Choroid plexus in lateral ventricles, neuropil, and neuronal changes were evaluated in coronal sections of the brain. The changes were different depending on survival days regardless of dosage of irradiation or cisplatin treatment. At 4 days after irradiation, the changes of choroid plexus were marked and associated with diffuse perivascular edema. At 6 days after irradiation, choroid plexus showed congestion of blood vessels with no changes in epithelial cells and neuropil.The degenerative changes in neurons were marked at 6 days and minimal after 7 days, which suggests the reversible changes of neurons in radiation damage of rat brain. Those rats with 20 Gy irradiation, killed at 3 or 9 days, showed mild to moderate vacuolizatlon of epithelial cells and dilatation of capillaries with no perivascular edema. Cisplatin treatment showed mild to moderate changes in choroid plexus with no changes in neurons and neuropil.