At the sessions of the Section on Pathology and Physiology in 1940 the word hypoxia was used in a symposium on anoxia to designate milder degrees of deprivation of oxygen. In the present discussion the word hypoxia1 will be used to mean any reduction in the tension of oxygen which produces disturbances of function only while the reduction persists. Hypoxia has completely reversible effects. Anoxia will mean a reduction of tension of such a degree that it is followed by changes of function persisting after the lowered tension is relieved. Anoxia is characterized by irreversible effects. Admittedly, an immediate decision cannot always be made as to which word, thus defined, ought to be used on a particular occasion. The use of these two terms does, nevertheless, distinguish between a condition of only temporary moment and one causing prolonged inactivity or death of some or all of the cells of