Differential hypothermia (D.H.) treatment (preservation of brain tumor at normothermia during generalized body hypothermia) for the treatment of human brain tumors requires the tumor to be heated throughly and maintained for the necessary time. The normal portion of brain has to be protected during and after the treatment. Microwave (MW) seemed the most suitable heating method, so its effects on the normal brain of dogs were studied. During MW irradiation, the thermistor-probe showed higher temperatures erroneously. When the probe was covered by aluminium foil, the measurement error was less than 0.2℃ as long as the probe was in the brain. This method sufficiently heated the brain under generalized hypothermia down to 2 cm in depth from the surface. The highest temperature was recorded between 0.5 cm and 1 cm in depth and was easily controlled by the output of MW. On the other hand, aluminium foil blocked MW irradiation completely, therefore the irradiation field possibly protected the normal portion. To study the safety limits of heating by MW irradiation, two groups of rabbits maintained at different temperatures (37℃ and 40℃) were prepared. Following D.H. treatment for 5 hours in each group, the rabbit brains were examined histologically. The results showed minimal brain damage by MW irradiation in the 37℃ group. In the 40℃ group, heat necrosis was constantly observed as well as stagnation of vessels and perivascular cuffing of round cells. From these findings, it was concluded that MW was useful as a heating method during D.H. treatment. The safety limit of heating was up to normothermia.