An interstitial microwave antenna system was devised for differential hypothermia treatment. It was evaluated for its ability to induce localized brain hyperthermia in hypothermic monkey. Ten brain hyperthermia trials have been performed in 6 monkeys. Under general anesthesia, the animals were put into ice water bath to keep the total body temperature at 30 degrees C. Following parieto-occipital craniectomy, a microwave antenna of 1.5 mm in diameter was inserted into the brain at depth of 2 cm, and the brain tissue was heated by 2450 MHz microwave irradiation. Thermal distribution was measured by thermistor probes and local cerebral blood flow (1-CBF) before and after heating was simultaneously measured by hydrogen clearance method. After the experiment, the animals were sacrificed and histopathological changes of the heated brain tissue were studied. Under total body hypothermia of 30 degrees C, the maximum cross-sectional diameter of the heated brain to 37 degrees C or above was about 4 cm. The temperature profile on the vertical plane presented a bell-shaped distribution. The 1-CBF of the heated brain increased with the elevation of the brain temperature and the blood flow at 37 degrees C is nearly twice as much as that of 30 degrees C. After one hour DH treatment, necrotic tissue was noted along the antenna axis where the temperature was maintained more than 50 degrees C, and this change was not recognized at a distance of 1 cm from the antenna where the temperature was maintained at 42 degrees C. This study indicates that interstitial microwave hyperthermia system can be used effectively to heat the localized brain tissue.