Objective Recent studies showed that change in plasma and cerebrospinal fluid (CSF) level of S100B protein was closely related to cerebral damage. The aim of this study was to investigate if deliberate hypotension induced by isoflurane can increase the release of S100B protein in CSF during clipping of intracranial aneurysm.Methods Thirty ASA Ⅰ - Ⅱ patients (16 male, 14 female) aged 24-68 yr undergoing elective intracranial aneurysm clipping were randomly divided into two groups : group A deliberate hypotension ( n = 15) and group B control ( n = 15) in which BP was maintained at normal level during operation. In both groups anesthesia was induced with midazolam 0.06 mg? kg-1 , fentanyl 3-5 ?g? kg-1 , propofol 2 mg? kg-1 and vecuronium 0.1 mg ? kg-1 and maintained with 1.2% isoflurane and intermittent intravenous (i. v.) blouses of fentanyl and vecuronium. After tracheal intubation the patients were mechanically ventilated (VT 8-10 ml?kg-1 , RR 12 bpm, I :E = 1:2). PaCO, was maintained at 35-40 mm Hg, In group A deliberate hypotension was induced by increasing the inhaled concentration of isoflurane until BP was reduced by 30 % -40 % of the baseline value. After clipping of aneurysm MAP was restored to baseline level. CSF level of S100B protein was measured before deliberate hypotension and 0, 2, 4 h after aneurysm clipping. Results (1) MAP was decreased from (95 ? 12) mm Hg to (59 ? 5) mm Hg 30 min after deliberate hypotension was started and restored to (75 ? 8) mm Hg 30 min after aneurysm was clipped. In group A both systemic peripheral vascular resistance and myocardial contraction acceleration were decreased but cardiac output and HR remained stable, as compared with those in group B. (2) CSF level of S100B protein was significantly increased 4 h after aneuuysm clipping in both groups but CSF level of S100B protein was significantly higher in group A than that in group B. Conclusion Deliberate hypotension induced by isoflurane increases the release of S100B protein and may worsen cerebral vasospasm and be detrimental to perioperative cerebral protection.