A state of lysosomal membranes and lipid peroxidation were studied in rat lungs under hypoxic conditions at the height 9,000 m, 3 hrs. Under these condition content of protein in lungs was decreased by 12% but free activity of lysosomal peptide hydrolases was simultaneously increased by 66%, indicating the lysosomal membranes labilization. At the same time, content of lipid peroxidation products--diene conjugates and Schiff bases, was decreased in lung synchronously with activation of the antioxidant enzymes catalase and superoxide anion dismutase. The lung tissue appears to have more powerful mechanisms of adaptation to deficiency of oxygen as compared with other tissues. Under hypoxic conditions content of protein in blood serum was similar to that of control values, while activity of lysosomal enzymes was increased by 26%, which occurred apparently due to an increase in the rate of lipid peroxidation. Actually, amount of lipid peroxidation products was increased in erythrocyte membranes but activity of SOD was not altered and the activity of catalase--tended to decrease. Thus, one of possible reasons of lipid peroxidation product accumulation in blood may be a lowered activity of the antioxidative enzymes.