In this paper the importance of constancy in the internal environment for the stability and well being of the organism as a whole and the cerebral cortex in particular has been emphasized. Numerous illustrations were cited from studies at high altitude to demonstrate the basic rôle of oxygen in neural activity. The way in which psychological functions can be altered by changes in the oxygen tension was illustrated by reference to specific experiments in neuro-muscular control, memory and light sensitivity. The results showed in striking fashion how sensitive psycho-physiological processes are to oxygen changes in the internal environment. The application of these observations and experimental findings were then discussed in relation to certain mental disorders. By placing these familiar psychological and psychiatric problems in this new setting they are seen in broader perspective for further study.Abstract Teaser