An effect of hypoxic hypoxia on the total, Mg2+-dependent and Na-K-dependent ATPase activities was studied in rat brain and erythrocytes. In brain tissue under effect of 15 min hypoxia a distinct decrease in all the ATPase activities was observed; the activity of Na-K-dependent ATPase decreased especially distinctly (by 32% of the initial level). Within 90 min of hypoxia the enzymatic activity was partially restored but did not reach the initial level. In erythrocytes 15 min hypoxia was also accompanied by a significant decrease in Na-K-dependent ATPase activity (30.5% of the initial level). But within 90 min the enzymatic activity was distinctly increased (up to 196.4%). Mg2+-dependent ATP-ase was more resistant to oxygen deficiency, Variations in the enzymatic activity were less distinct, although they showed the same pattern as those of the Na-K-dependent ATPase.