The dynamic EEG study was performed in 19 patients with mesencephalic damages (verified by the contrast methods and during the surgery). A new EEG phenomenon, mesencephalic waves (MW), was observed in the early postsurgery period during the development of acute excitation focus. MW were recorded predominantly in the frontal and temporal neocortical areas. They appeared as a potential complex consisting of a negative monophasic wave with a superimposed one- or two-phasic spike followed by a 2-3 second theta-delta-burst. MW were recorded in 11 patients of 19 with mesencephalic damages and were not observed in the EEG of the control patients.