Differential hypothermia (D.H.), in which tumors are kept normothermic under total body hypothermia, has been confirmed to have a therapeutic effect. It is essential in the treatment of malignant brain tumors by D.H. to deliver the heat uniformly to specific volumes of tissue and to maintain the optimal temperature for an appropriate period of time. A new instrument for delivering electromagnetic radiation at 915 MHz was devised, and its therapeutic efficacy was evaluated in comparison with conventional 2,450 MHz microwave diathermy. Adult canine or monkey brains were exposed to microwave irradiation following fronto-parietal craniectomy under total body hypothermia (29±1℃). The highest temperture in the brain was controlled manually to be between 3-37℃. The average temperature at depths of 0, 0.5, 1.0, 1.5, 2.0 and 3.0 cm was 34.8℃, 35.5℃, 36.0℃, 36.7℃, 34.4℃, and 30.7℃, respectively, at 915 MHz ang 3.8℃, 35.4℃, 36.0℃, 3.℃, 34.5℃, and 32.0℃, respectively, at 2,450 MHz. Deeper heat penetration was achieved with 915 MHz than with 2,450 MHz MW irradiation. The temperature of the brain was controlled easily and could be kept 7-10℃ higher than that of the cooled body. Heat toxicity was examined histopathologically following a 5-hour-D.H. treatment. No brain damage was observed in brains heated to 37℃, but irreversible changes such as exudative homorrhage were revealed in brains kept at 40℃. These results suggest that : 1) 815 MHz MW irradiation can effectively induce localized brain hyperthermia. 2) Deeper heat penetration was achieved by 915 MHz than 2,450 MH MW irradiation. 3) Temperatures above 40℃ for 5 hours induced irreversible changes in the brain under total body hypothermia (29±1℃).