Dexamethasone was administered to mice bearing intracerebral ependymoblastomas. Maximum increased survival of 24% over nontreated animals required a dose of 40 mg/kg daily beginning the day following tumor implantation. Water and sodium content of brain adjacent to and distant from the tumors was reduced by lesser doses of dexamethasone as well. The tumors in the treated mice were substantially smaller than those in nontreated controls, and the drug retarded the incorporation of tritiated thymidine into tumor DNA. The mechanisms by which corticosteroid hormones reduce edema surrounding brain tumors and inhibit tumor growth remain unknown.