Deliberate hypotension was induced with isofiurane (mean inspired concentration 2.3 ± 1.0%) in 12 patients undergoing craniotomy for clipping of cerebral aneurysms. Global cerebral blood flow (CBF) was measured before, during, and after hypotension. Arterio-venous O2content difference was measured concomitantly, and the cerebral metabolic rate for oxygen (CMRO1)was calculated from these data. Mean arterial pressure (MAP) was reduced from 78 ± 5 mmHg to 51 ± 7 mmHg and then returned to 82 ± 8 mmHg. Mean CBF before hypotension was 49 ± 14 ml·100 g−1-min−1and was unchanged during (45 ± 12 ml · 100 g−1· min−1) and after (49 ± 15 ml. 100 g−1· min−1) hypotension. The CMRO1before hypotension was 2.0 ± 0.6 ml · 100 g−1· min−1. This was statistically significantly (P < 0.025) reduced to 1.5 ± 0.5 ml · 100 g−1· min−1during hypotension and then returned to 2.2 ± 0.6 ml. 100 g−1· min−1on return to normotension. This indicates that the global cerebral O2supply-demand balance was favorably influenced by isoflurane. No complications could be attributed to the hypotensive technique. We conclude that, with regard to global cerebral oxygenation, isoflurane is a safe agent with which to induce hypotension during neurosurgery.