Hyperthermia in conjunction with brachytherapy might allow the delivery of less toxic radiation doses while improving local control of tumors. To complement the authors' clinical study of interstitial brachytherapy of malignant brain tumors, a microwave interstitial heating system with computed data acquisition was constructed. It was evaluated in a phantom and in a series of acute and chronic experiments for heating patterns and heat toxicity in 30 normal adult canine brains. With the head fixed in a stereotaxic frame, lateral, occipital, or parietal approaches were used for placement of nylon catheters containing microwave antennas and fiberoptic thermometry probes. The antennas were operated at 915 or 2, 450 MHz using continuous-wave power sources. When a steady state was obtained, usually within 5 minutes of heating, temperatures were mapped at 1 mm intervals along the length of the antenna and radially from the antenna by moving the thermometry probes with a stereotaxic manipulator. In general it was easy to maintain a steady state for a long period. Heating patterns tended to be ellipsoidal, correlating relatively well with those predicted by phantom trials. Heat toxicity was examined by sequential quantitative computed tomography (CT) scans in chronic trials and correlated with histopathology following sacrifice at 4-8 weeks. The results suggest that 1) interstitial microwave hyperthermia is an effective means of inducing localized brain hyperthermia, 2) surgical and hyperthermia procedures are well tolerated, 3) temperature above 43°C for 30 minutes induces changes in the normal canine brain which can be measured and followed noninvasively with CT.