We investigated the temperature changes and their distribution in agar phantoms and dog normal brains induced by 8 MHz radiofrequency interstitial hyperthermia and observed the histological changes, with respect to the neurons and myelinated nerve fibres, induced by the same heat source in dog normal brains. We also examined the change of blood-brain barrier permeability using Evans blue solution. The heating limits of dog normal brain were 42 degrees C for 45 min or 43 degrees C for 15 min and the breakdown of the BBB was observed at 43 degrees C for 60 min.