The purpose of the investigation was to study the electrophysiological manifestations of arterial spasm developing following surgical interventions for cerebral arterial aneurysms. Repeated EEG-recording was undertaken for this purpose in 40 patients with aneurysms of different localization. The EEG data were compared with the results of a neurological examination and angiography performed pre- and postoperatively. It was found that with an uneventful postoperative course no focal changes develop in the bioelectrical activity, and EEG returns to the initial (preoperative) state or to the normal pattern as soon as by the 15th-20th day. In the presence of postoperative arterial spasm, verified clinically and angiographically, the latter is reflected in EEG tracings as peculiar focal changes in the biopotentials. These changes have a nature of a periodically occurring delta-activity recorded predominantly in the zone of ischaemia. Along with a regress of the spasm its EEG manifestations were found to decrease as well. The focal EEG changes observed in postoperative spasm differ from the localization of those EGG changes that are caused by other factors (haematoma, cicatrical processes, etc.).