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Vitamin C concentrations in the brain exceed those in blood by 10-fold. In both tissues, the vitamin is present primarily in the reduced form, ascorbic acid. We identified the chemi-cal form of vitamin C that readily crosses the blood–brain barrier, and the mechanism of this process. Ascorbic acid was not able to cross the blood–brain barrier in our studies. In contrast, the oxidized form of vitamin C, dehydroascor-bic acid (oxidized ascorbic acid), readily entered the brain and was retained in the brain tissue in the form of ascorbic acid. Transport of dehydroascorbic acid into the brain was inhibited b