The activity of cytochrome oxidase in tissues was decreased in short-term adaptation to reduce pO2 and administration of KCN in small doses. In the former case the activities of peroxidase and catalase were increased in blood. These effects as well as administration of Na2ATP into rats led to an increase in water content in tissues, to soption of acidic vital stain (phenol red) and to decrease in the ether-soluble fraction of lipids. The alterations were accompanied by decreased permeability of cells to n-hexane (estimation by gas-liquid chromatography). The decrease in cell permeability for nonelectrolytes was apparently due to conformational alterations in protein molecule of cytoplasmic membranes.