Isoflurane has protective properties during experimental global brain ischemia or hypoxia. However, this has not been evaluated in the more common case of focal ischemia, e.g., as caused by middle cerebral artery occlusion (MCAO). The authors therefore compared the effects of isoflurane, thiopental, and N2O/fentanyl anesthesia on neurologic and neuropathologic outcome in baboons subjected to 6 h of transorbital left MCAO. Prior to MCAO, animals were assigned to one of three groups: Group 1 (n = 7) received isoflurane (in O2/air) in concentrations sufficient to maintain deep burst suppression on the EEG (2.0% ± 0.5% inspired, mean ± SD); group 2 (n = 6) received thiopental (O2/air) in doses adequate to maintain similar EEG suppression (3.6 ± 0.7 g total); and group 3 (n = 6) received 60% N2O/40% O2 and fentanyl (25 μg/kg load, 3 μg·kg-1·h-1 infusion). Efforts were made to keep mean arterial pressure (MABP) between ~ 80 and 100 mmHg, using nitroprusside/hydralazine or phenylephrine/metaraminol, with Pa(CO2) at ~ 30 mmHg. The selected anesthetic was established 45 min before MCAO, was maintained until 1 h after clip removal, and in decreasing concentrations for 5 h. Neurologic status was scored for 7 days and formalin-fixed brains were later sectioned for determination of infarction volume. Six of seven group 1 (isoflurane) animals were hemiplegic, and 7/7 had verified infarctions. By contrast, 4 of 6 group 2 (thiopental) animals were normal, with 2/6 having infarctions. Outcome in group 3 (N2O/fentanyl) was intermediate between groups 1 and 2 (3/6 hemiplegic, 4/6 with infarctions). Differences in the infarction rates between groups 1 and 2 was significant (P < 0.05), while a similar comparison of neurologic outcome scores achieved a P value of 0.055. Infarctions in group 1 were more hemorrhagic in character than in group 3 (groups 1 and 2 could not be meaningfully compared). These results must be considered in light of differences in MABP during the occlusion period; MABP in group 1 was ~ 80 mmHg in spite of vasopressor use, while that in group 2 was ~ 100 mmHg (in spite of vasodilators). Nevertheless, they fail to demonstrate any protective value of isoflurane anesthesia, at least when compared with thiopental.