Changes in plasma and RBC alpha-tocopherol after intravenous and oral administration of dl-alpha-tocopheryl acetate were investigated using vitamin E-deficient rats and were compared with those following the administration of d-alpha-tocopherol, which is the most active and natural form of tocopherol. Intravenous administration: By the administration of dl-tocopheryl acetate, the elevated plasma tocopherol levels were higher at 6 h than those elevated by d-tocopherol, but the RBC tocopherol was lower 6 h after the infusion of dl-tocopheryl acetate than after d-tocopherol infusion. The tocopheryl acetate incorporated into the RBCs remained for 24 h after the administration of tocopheryl acetate, the acetate then becoming undetectable in plasma. Oral administration: The tocopherol elevated in plasma and RBC was only the alpha-form and not tocopheryl acetate. In both plasma and RBCs, tocopherol levels were higher after the administration of d-tocopherol than after dl-tocopheryl acetate administration, and more was found in RBCs.