Somatosensory evoked potentials (SEP) were used intraoperatively 25 cerebral aneurysm cases during the temporary occlusion of the parent artery of the aneurysm under moderate hypothermia. This technique of vascular occlusion is very useful in facilitating the dissection of difficult aneurysms as well as in reducing the risk of intraoperative rupture. Middle cerebral artery (MCA) cases in Hunt & Hess's grade III, undergoing early surgery, who had shown a transient neurological deficit at the time of subarachnoid haemorrhage or where vasospasm was evident in intraoperatively were prominent among 6 cases where the median nerve SEP was lost within 13 minutes of temporary MCA occlusion at 28.6 degrees C to 31.1 degrees C. A transient neurological deficit was seen in one of these and a permanently increased deficit in the other. In contrast, the SEP was well maintained during occlusion times of upto 52 minutes in 8 cases in the absence of any of the above circumstances. The SEP was lost after 7 minutes in one of 5 cases of internal carotid artery occlusion; this was followed by a paresis of a few hours' duration. The posterior tibial nerve SEP was absent for one minute in one of 5 cases of bilateral A1 segment occlusion; none of these cases showed a postoperative deficit. It is concluded that 1. appropriate SEP monitoring can make a major contribution to patient safety in aneurysm surgery, 2. substantially longer cerebrovascular occlusion times are permissible during hypothermia than at normal temperatures and 3. the employment of additional cerebral protective measures should be considered in cases at high risk from ischaemic damage.