To investigate the role of cerebral hypoxia as a causative factor in the alteration of the qEEG during hyperventilation, qEEG changes caused by progressive hypocapnia were compared with qEEG changes due to progressive normobaric hypoxia in two parallel groups of 12 and 10 healthy male subjects (age 20-27 years), respectively. In the first group, qEEG records were obtained before and during hyperventilation to pCO2 levels of 4.0, 3.0 and 2.0 kPa. In the second group, the qEEG samples were taken before and during hypoxia with hemoglobin oxygen saturations of 80, 70 and 60%. In both groups, blood flow velocity in the middle cerebral artery was also recorded. Hyperventilation caused an exponential increase in slow activity and a decrease in alpha power. No shift in the alpha mean frequency and alpha peak frequency was observed, except with the pCO2 level of 4.0 kPa, which caused an increase in both variables. Hypoxia with a hemoglobin oxygen saturation of 60% caused a much less pronounced increase in slow activity. No change in total power in the alpha band was found, but both the alpha peak frequency and alpha mean frequency decreased. Lesser degrees of hypoxia caused only minimal EEG changes. Blood flow velocity was decreased by hyperventilation but increased by hypoxia. It is concluded that the EEG changes observed during hyperventilation must mainly or totally be attributed to factors other than cerebral hypoxia.