In practice of systemic or local hyperthermia, it becomes very important to determine the tolerance of the brain to heat, along with the evaluation of the influence of heat on brain function. In this little known area, the study was made by examining the changes of blood pressure, cerebral blood flow (CBF), partial pressure of oxygen and pH in the cerebral cortex, electroencephalogram (EEG) and somatosensory evoked potential (SEP), under hyperthermia by experiment. Unanesthetized and immobilized adult rabbits were used in the experiments under controlled respiration. The head alone or the whole body was heated by bathing in hot air. In the head heating, the brain temperature could be heated up to 45 degrees C without extreme fall of the blood pressure, and meantime the aortic blood maintained under 43.5 degrees C at that condition. In the head heating case, the CBF increased to two times at 43 degrees C, two and a half times at 44 degrees C, three and a half times at 45 degrees C of the cortical temperature. As the cortical temperature raised up, the partial pressure of oxygen and pH of the cerebral cortex increased. The peak frequency of power spectrum of EEG increased as the temperature was raised. High amplitude rhythmic slow wave burst appeared at over 44 degrees C for 30 minutes, which soon changed to a flat pattern. In the whole body bathing, on the contrary, the animals died of abrupt fall of the arterial blood pressure when the temperature of the aortic blood was raised over 43.5 degrees C, and it was impossible to get the brain heated over 43 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)