To clarify the permissible time for temporary vascular occlusion during aneurysm surgery, we have undertaken a retrospective analysis of the results of intraoperative median nerve somatosensory evoked potential (SEP) monitoring in 97 patients with middle cerebral artery (MCA) (46 patients) or internal carotid artery (ICA) (51 patients) aneurysms. All patients underwent temporary vascular occlusion, lasting from 2 to 70 minutes, under the administration of a mixed solution of 20% mannitol with phenytoin and vitamin E. The mean occlusion time was 20.3 minutes for the MCA patients and 15.8 minutes for the ICA patients. The SEP disappeared during occlusion in 42 patients (30 MCA and 12 ICA). All but three eventually recovered the SEP to the baseline level after recirculation, and none of the 39 patients had postoperative sequelae. The time period from the start of occlusion until the complete loss of the SEP averaged 8.6 minutes among these 39 patients, and the occlusion time from total SEP loss until recirculation averaged 12 minutes. In the remaining 3 of the 42 patients (all 3 being ICA aneurysm patients), however, the SEP did not recover after recirculation and all 3 patients showed postoperative sequelae. In two of these three patients, vascular occlusions were performed at the multiple sites (i.e., at the ICA, MCA, anterior cerebral artery, and posterior communicating artery) and the SEP disappeared rapidly after the occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)