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Previous work has demonstrated that rats anesthetized with halothane during focal cerebral ischemia have better histologic and neurologic outcome than do rats undergoing the same insult when awake. The purpose of this experiment was to determine whether this difference persists when brain temperature is held similar in halothane- anesthetized and awake experimental groups.Two ischemia experiments were performed. In both, the middle cerebral artery was occluded for 90 min. Temperature was monitored from a radiotelemetered thermistor implanted in the cerebral cortex. Four days after ischemia, infarct volume and neurologic function were assessed. In experiment 1, brain temperature was not controlled in awake rats. Temperature in rats anesthetized with halothane, approximately 1 minimum alveolar concentration, was regulated by servomechanism by surface heating or cooling to replicate the temperature profiles generated by awake animals. To address methodologic issues regarding infarct volume analysis, a subset of nine rats was examined for the effect of the histologic staining technique and the mathematical modeling algorithms used for computation of infarct volume values. In experiment 2, the brain temperature of awake and halothane- anesthetized rats was maintained normothermic (38.0 degrees Celsius) throughout ischemia and early recirculation.In experiment 1 no difference between groups was observed for cortical (halothane 146 plus/minus 95 mm3 and awake 126 plus/minus 108 mm3; P = 0.64) or subcortical (halothane 110 plus/minus 48 mm3 and awake 100 plus/minus 66 mm3; P 0.66) infarct volume. Neurologic function was also similar between groups. Total infarct volume was approximately 11% greater when histologic sections were stained with hematoxylin and eosin than when they were stained with nitro blue tetrazolium, although volumes correlated closely between the two techniques (r2 0.996). Analysis by orthogonal or frustum projection from two-dimensional planimetric areas to three- dimensional volumes resulted in nearly identical values (r2 0.999). In experiment 2, halothane-anesthetized rats experienced a 46% reduction in cortical infarct volume (halothane 106 plus/minus 97 mm3 and awake 197 plus/minus 103 mm3; P = 0.03). The incidence of hemiparesis was reduced in the anesthetized group (P = 0.03).When brain temperature was maintained normothermic throughout the focal ischemic insult, a neurologic and histologic protective effect for halothane anesthesia was observed. This effect of halothane was not sufficient to persist when large variations in brain temperature were allowed. Regulation of brain temperature is a critical factor in the determination of the effects of anesthetics on focal ischemic brain damage.