1. (1) EEG test were carried out in Morococha (4,540 m above sea level) on eight normal subjects who were born and were living above 3,600 m altitude. 2. (2) In Magdalena (district of Lima at sea level) five of the subjects were examined during the 1st month and between the 2nd and 3rd month after their descent. A last EEG was recorded for four of the subjects between the 6th and 7th month after their arrival in Lima. Blood sugar determinations and the analysis of alveolar air were made for some of the subjects at or near the time of the EEG recordings. 3. (3) The recordings during rest at high altitude were within normal limits. When the group descended from Morococha to Lima, an increase was found in the voltage of the background activity with a statistically significant decrease in the mean frequency of the parieto-occipital rhythm (from 11 to 10.5 c/sec). Concomitantly there was an increase in voltage and in quantity of the slower waves. 4. (4) No statistically significant correlations have been found between the EEG frequency and the alveolar oxygen and carbon dioxide pressures and the pH values. 5. (5) Of the 16 recordings made in Morococha, only one presented slow waves during hyperventilation whereas all 14 EEGs of the subjects in Lima showed slow waves during voluntary hyperpnea. 6. (6) The stability of the EEG of highlanders in Morococha would indicate an acclimatization to high altitude. 7. (7) The EEG changes found upon descent could be considered as one indicator of the processes of acclimatization of the highlander to sea level conditions. 8. (8) The findings are discussed and possible interpretations are offered. © 1965.