Adaptation of animals to repeated immobilization stress is shown to stimulate development of super-resistivity to severe hypoxic hypoxia. This phenomenon manifests itself by the fact that breathing hypoxic gas mixture containing 60% O2 causes 65% mortality among control rats and only 10% mortality among adapted animals within 2 hrs of experiment. Inquiry into mechanisms responsible for this multiple rise in resistivity showed that adaptation to stress increases rat tolerance of hypoxia due to economization of breathing and circulation, considerable strengthening of the tissue abilities to take up O2 from blood despite expressed hypoxemia, and due to limited acidosis. Adaptation also significantly suppresses hypoxia-induced processes of damaging cellular membranes, namely, activation of lipolysis and lipid peroxidation. The role of stress-limiting systems and stabilization of structures in the cross defense effect of adaptation to stress is discussed.