1. Observations on the effect of breathing low oxygen are reported in a series of 67 patients and 42 controls, in whom changes in central nervous system activity were measured by the EEG and changes in behavior by the interaction chronogram.2. Most subjects showed a shift in EEG to the slow frequencies during low oxygen. This was more marked and consistent when the CO₂ was less than .3%. There was no difference between patients and controls.3. A decrease in verbal and gestural activity was found in about three-fourths of the subjects. The decrease was greater for the patients than the controls.4. Subjects with high initial activity while breathing air showed the greatest decrease as a result of low O₂. Subjects with low activity rates in air showed slight decreases and, in many instances, an increase.5. Subjects with marked changes in EEG tended to show an increase in activity.6. Four factors are discussed as playing a rôle in determining the changes in activity due to low O₂.(a) Initial rate of activity.(b) Diagnosis.(c) Electroencephalographic changes.(d) Autonomic activity as measured by heart rate.Abstract Teaser