Rats of three age groups were fed tocopherol deficient or supplemented diets for 16 weeks or until signs of tocopherol deficiency were apparent. Erythrocyte hemolysis and liver tocopherol content were used as measurements of the tocopherol status of the rats. The following measurements were made on liver microsomal and mitochondrial fractions of all three groups; phospholipid content, lipid peroxidation, fatty acid patterns, pigment fluorescence, ANS fluorescence and the activities of several membrane bound enzymes. Eleven week-old rats displayed signs of vitamin E deficiency after consuming the diet for 7 weeks. Forty-two-week-old rats displayed borderline deficiency signs after 16 weeks of consuming the diet whereas 67-week-old rats displayed no deficiency signs. The need for dietary tocopherol, therefore, appeared to decrease with increasing animal age. Age related alterations in membrane compositional and functional parameters were not modified by either tocopherol deficient or supplemented diets. Tocopherol does not appear to stabilize microsomal membrane composition or function although mitochondrial membranes appear to be labilized by the dietary manipulation of the vitamin.