A patient is reported in whom intraoperative somatosensory evoked potential (SEP) changes occurred in response to temporary clipping of the right middle cerebral artery. A period of 10 minutes elapsed before changes in SEP's in response to contralateral nerve stimulation were noted and, during the following 2 minutes, the waves decreased in amplitude and then were unrecordable. Waves of SEP, with amplitude similar to those recorded before clipping but with abnormal latency, returned within 45 seconds of removal of the clip, and the latency abnormalities persisted until the end of the operation. The patient awakened promptly at the end of the procedure with a dense left hemiparesis which resolved over 24 hours. At the end of 24 hours, the SEP's in response to median nerve stimulation were symmetrical in both latency and amplitude. This report demonstrates the accuracy of intraoperative evoked potential monitoring in demonstrating alterations of cerebral perfusion during aneurysm surgery. It also suggests that a prolonged period of observation may be necessary to assess the effects of temporary vessel occlusion during surgery on aneurysms or arteriovenous malformations.