A systematic study was undertaken to investigate to what degree the inhalation of CO₂ combats the effects of a certain degree of O₂-deficiency on more complex cortical processes. It was shown that the typical effect of O₂-deficiency on the association process, consisting in the increased number of individual responses and the occurrence of perseverations and dissociations, is completely absent in the same experimental subjects when the same O₂ concentration is inhaled in the presence of 3 per cent CO₂. The results with regard to memory are similar. Memory is greatly reduced under O₂-deficiency, but practically normal in the corresponding experiment involving the same degree of O₂-deficiency +3 per cent CO₂. Severe changes in handwriting and misspelling occurring under O₂-deficiency are described. They are absent if the same O₂ concentration + 3 per cent CO₂ is inhaled. The time needed to carry out the number cancellation and addition tests is increased under O₂-deficiency but no such change occurs if under the same circumstances 3 per cent CO₂ is inhaled together with the oxygen-deficient air-N₂-mixture.In general, it is found that memory is the most sensitive process suffering from O₂-deficiency. As to the relationship to each other of the other processes studied, it may be stated that occasionally muscular coordination, as shown by the handwriting test, is greatly interfered with in spite of the fact that the associative response appears unchanged, whereas in other subjects the disturbances in associations are more pronounced. The characteristic subjective phenomena observed under O₂-deficiency are more or less absent in the corresponding O₂-deficiency experiment with 3 per cent CO₂.The experiments prove conclusively that the complex "psychic" processes described in this paper depend on, and are modified by, the same physiologic factors which determine ordinary physiologic functions.Abstract Teaser