Regional cerebral blood flow (CBF) was measured (intra-arterial injections of 133Xe) and electroencephalograms (EEG) were recorded periodically before, for two hours during, and for one and one-fourth hours after middle cerebral artery (MCA) occlusion in 20 squirrel monkeys (Saimiri sciureus). A CBF-Paco2 response curve for these animals under barbiturate anesthesia was created from CBF values prior to MCA occlusion and during the time a steady state was being achieved. The animals were subdivided into four groups (five monkeys in each) on the basis of Paco 2 values: 20, 36, 40, and 60 mm Hg. CBF values from this study were compared to previous results obtained with 85Kr. The phenomenon of "look through" and the importance of recognizing this artifact and its significance in analyzing CBF results in areas of focal ischemia are discussed. The present results were correlated with cerebral ATP and lactate concentrations in ischemic regions determined in previous studies using this preparation at these Paco2 values and at comparable time intervals before, during, and after MCA occlusion. The EEG appears to reflect the state of ischemic brain accurately. However, CBF measured by the 133Xe method can be misleading in regard to the true degree of ischemia resulting from occlusion of an intracranial vessel and cannot be relied on to demonstrate accurately "steal" or "reverse steal" due to changes in Paco2.