Somatosensory evoked potentials have been recorded during 34 operations for intracranial aneurysm. The central conduction time (CCT), the time between the N14 peak (recorded at C-2) and the N20 peak (recorded at the cortex) in response to median nerve stimulation, has been found to be increased by administration of halothane, by brain retraction, and by temporary vascular occlusion in some instances. Increase of CCT to more than 10 msec, or disappearance of the response was associated in two cases with postoperative neurological deficit, neither permanent. In a further case, prolongation of CCT was used as a guide to the repositioning of an imperfectly placed clip on a middle cerebral artery aneurysm. The value of the technique as a perioperative monitoring system is discussed.