The reversibility of changes in ultrastructure, K+, and ATP content was studied in experimental injury of Ehrlich ascites tumor cells. Different grades of injury resulted from incubations in N2 atmosphere and omitting substrate after which air and glucose were reinstated. The changes observed in cells after 1 hour of anoxia such as dilations of endoplasmic reticulum, complex invaginations of plasma membrane, and slight condensation of mitochondria, as well as a drop of K+ and ATP content to a level approximating 40 per cent of the paired controls, were entirely reversible. After 2 hours of anoxia approximately 50 per cent of the cells recovered, but after 3 and 4 hour of anoxia most of the cells were irreversibly damaged showing markedly swollen mitochondria with flocculent densities.