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Propofol and isoflurane anesthesia were studied previously with functional brain imaging in humans to begin identifying key brain areas involved with mediating anesthetic-induced unconsciousness. The authors describe an additional positron emission tomography study of halothane's in vivo cerebral metabolic effects.Five male volunteers each underwent two positron emission tomography scans. One scan assessed awake-baseline metabolism, and the other scan assessed metabolism during halothane anesthesia titrated to the point of unresponsiveness (mean +/− SD, expired = 0.7 +/− 0.2%). Scans were obtained using a GE2048 scanner and the F-18 fluorodeoxyglucose technique. Regions of interest were analyzed for changes in both absolute and relative glucose metabolism. In addition, relative changes in metabolism were evaluated using statistical parametric mapping.Awake whole-brain metabolism averaged 6.3 +/− 1.2 mg [middle dot] 100 g-1 [middle dot] min-1 (mean +/− SD). Halothane reduced metabolism 40 +/− 9% to 3.7 +/− 0.6 mg [middle dot] 100 g-1 [middle dot] min-1 (P Halothane caused a global whole-brain metabolic reduction with significant shifts in regional metabolism. Comparisons with previous studies reveal similar absolute and relative metabolic effects for halothane and isoflurane. Propofol, however, was associated with larger absolute metabolic reductions, suppression of relative cortical metabolism more than either inhalational agent, and significantly less suppression of relative basal ganglia and midbrain metabolism.