Background and Purpose: Despite the documented diagnostic value of local cerebral blood flow maps by xenon-enhanced computed tomography, reports of cerebral blood flow activation by inhaled 33% Xe raised concerns about the method's safety and accuracy. We evaluated the effect of 33% Xe inhalation on cerebral blood flow and cerebral metabolic rates for oxygen and glucose in four awake and six fentanyl-anesthetized rhesus monkeys.Methods: Platinum microelectrodes and catheters in the torcular Herophili were used to measure cerebral blood flow by hydrogen clearance, and oxygen and glucose concentrations. Cerebral variables were measured after 5 and 35 minutes of exposure to room air followed randomly by 67% O 2 in 33% N 2 or Xe. Five-and 35-minute measurements were combined because the duration of exposure had no effect Results: In awake monkeys, 33% Xe compared with 33% N 2 reduced (p<0.05) cerebral blood flow from 75±12 to 66±9 (mean±SD) ml -100 g"' • min" 1 and oxygen consumption from 6.1±0.7 to 5.1±0.6 ml • 100 g" 1 • min"