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The body temperature was raised stepwise in rats to determine its effect on brain stem auditory evoked potentials. The underlying aim was to examine whether the resultant changes in evoked potentials could be used to predict both the regional temperature and functional state of the brain stem. Such data are likely to be of benefit in cases where hyperthermic therapy is being considered as an accompaniment to established therapies of brain stem gliomas. The data showed that over a rectal temperature range from 36.75 to 42.65 degrees C, corresponding to a brain stem temperature range of 35.25-41.5 degrees C, both amplitudes and latencies of components of the brain stem potentials systematically declined with temperature elevation. Measurements of brain stem temperature were highly correlated with interpeak latency measurements II-V and II-VI: the correlation coefficients were 0.87 and 0.84, and the 95% confidence intervals were +/- 0.54 degrees C and +/- 0.59 degrees C, respectively. Maintenance of rectal temperatures at 42 degrees C or at 42.5 degrees C for periods of, respectively, 30 min and 15 min produced a series of changes in the evoked potentials: latency increases, appearance of additional peaks, decreases in amplitudes, and losses of components V and VI. Hence, this technique may yield a means of predicting in a non-invasive manner both the regional temperature and functional state of the brain stem during hyperthermia.