During monitoring of somatosensory evoked potentials (SEP) during operation for intracranial aneurysm disappearance of cortical response can alarm the neurosurgeons and suggest the necessity of removal of temporary clipping. As a rule, 50% fall of amplitude and/or 10% prolongation of N13-N20 interlatence are accepted as a criterion for alarming the operating team. However, a considerable part of changes in SEP tracings is caused by factors other than surgical manipulations. The purpose of the present study was to determine the effect of general anaesthesia on SEP records for their further assessment during monitoring of operations for intracranial aneurysm. In 13 patients operated on for that reason and in 8 patients undergoing operation for lumbar disc prolapse SEP were monitored from over the Erb point, cervical cord (CVII) and parietal area on the side of the operation (C3' or C4'). Stimulation was electrical with stimulus duration 0.1 ms and 5 c/s frequency. General anaesthesia was found to decrease significantly the amplitude of the cortical wave, more evident in the N20-P25 amplitude, and amounting to about 40%. No statistically significant changes of the time of central conduction were found. The observed SEP changes made possible in all cases monitoring of operation for intracranial aneurysm.