It has been known for over 25 years that in most young and healthy subjects, hyperventilation produces a feeling of lightheadedness, difficulty in concentration, and high voltage slow waves in the electroencephalogram (EEG) but until the present time, the pathogenesis of the EEG slowing remains unproven.3Lennox, Gibbs, and Gibbs7considered that cerebral vasoconstriction occurring during hyperventilation maintained homeostasis of carbon dioxide (CO2) in the brain and that if cerebral PCO2levels became reduced, EEG slowing resulted. Davis and Wallace offered an alternative hypothesis that EEG slowing during hyperventilation was due to vasoconstriction with ischemic anoxia.2Concurrent recordings of alveolar CO2concentration and arterial oxygen saturation in man have shown that hypoxia in the absence of hypocapnia as well as hypocapnia in the absence of systemic hypoxia may produce EEG slowing.11On theoretical grounds, hyperventilation might produce EEG slowing either by ischemic hypoxia