Benefits from an antihypoxic agent (sodium oxibutyrate) and normal pressure hyperoxia to the lipid composition of erythrocyte membranes and blood plasma were assessed in 9 subjects on the thirtieth day of head-down tilting at -8(0). The greatest variations in HDT occurred in polyunsaturated fatty acids in erythrocyte membranes. After infusion of sodium oxibutyrate a marked reduction in polyunsaturated fatty acids was mainly in consequence of expressed decreases in linoleic and, particularly, arachidonic acids. However, ensuing hyperoxic treatment led to their steady rise. Sequential sodium oxibutyrate infusion to the tilted subjects breathing air and then exposed to normal pressure hyperoxia seemed to have compensated, as judged by the fatty acids in erythrocyte membranes, for the activity of free radical oxidation in hypoperfusated regional tissues.