Hypotension may be expected to produce less perturbation of metabolism in the brain when cerebral metabolic rate is lowered by deep anesthesia. Male Wistar rats having unilateral carotidartery ligation were exposed to mean arterial pressure (MAP) of 40 torr for 22 min by an intravenous infusion of trimethaphan during anesthesia with halothane, 0.6 or 2 per cent, in oxygen. Cortical tissue metabolite levels on the side of the ligated carotid artery were more abnormal in rats receiving halothane, 0.6 per cent, than in those receiving halothane, 2 per cent. Values at halothane, 0.6 per cent, were adenosine triphosphate (ATP), 1.71 +/- 0.05 (+/-SEM) mumol/g, phosphocreatine (PCr) 1.97 +/- 0.07 mumol/g. and lactate 16.5 +/- 5.1 mumol/g; corresponding values at halothane, 2 per cent, were ATP 2.27 +/- 0.02, PCr 4.02 +/- 0.23, and lactate 4.75 +/- 0.9 mumol/g. ATP and PCr values were significiantly lower (P less than 0.05) and the lactate value was significantly higher with halothane, 0.6 per cent, than with halothane 2 per cent. Cerebral oxygen consumption decreased 47 per cent in rats anesthetized with halothane, 2 per cent. Preservation of cortical metabolite levels in deeply anesthetized animals suggests a protective effect of cerebral metabolic depression.