This report deals with the study of electroencephalograms of 36 subjects who inhaled hypoxic mixture at the sea level (Po_2 is as that at 7000 m above sea level) or in a hypobaric chamber with rapid decompression (as that at 5000—7500 m above sea level) and of 46 subjects who reached high altitude area (5000—8600m) under chronic hypoxia. The characters of the EEGs at rest during photie stimulation, hyperventilation and convalescence after hypoxia were compared in two experiments. The result of experiments shows that in the same individual, the severer the hypoxia, the more obvious the changes on the EEGs. At the same altitude, acute hypoxia causes more obvious changes on the EEGs than chronic one does, but its speed of recovery is faster than that of chronic hypoxia. Under chronic hypoxia, the changes on the EEGs caused by hyperventilation are slighter than those under acute hypoxia. When photic stimulation is given, there is temporary appearance of a waves on the EEGs featuring slow waves under acute This report deals with the study of electroencephalograms of 36 subjects who inhaled hypoxic mixture at the sea level (Po_2 is as that at 7000 m above sea level) or in a hypobaric chamber with rapid decompression (as that at 5000-7500 m above sea level The characters of the EEGs at rest during photie stimulation, hyperventilation and convalescence after hypoxia were compared in two experiments. The result of protocols that that in the same the severer the hypoxia, the more obvious the changes on the EEGs. the more obvious the changes on the EEGs. the more obvious the changes on the EEGs. hypoxia, the changes on the EEGs caused by hyperventilation are slighter than those under acute hypoxia. when photic stimulation is given, there is temporary appearance of a waves on the EEGs featuring sl ow waves under acute