The effectiveness of heat therapy in combination with 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea on a murine ependymoblastoma was investigated. Based on survival time and the number of survivors, whole-body hyperthermia (40 degrees) increased the therapeutic effectiveness of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea. Heat alone did not modify the course of the tumor. Microscopic evidence of accelerated tumor destruction in hyperthermic mice was apparent within 24 hr of drug administration. A temporary drop in animal weight was observed with hyperthermia at the higher dose levels of drug. Mechanisms which may be involved in this synergism are discussed.