The conversion of alpha-tocopheryl quinone into alpha-tocopherol in humans has been demonstrated. A male subject was given an oral dose of 400 mg of alpha-3,5-[(C2H3)2]-tocopheryl quinone with an evening meal. Analysis of plasma 15 h later by lipid extraction and subsequent GC-MS single ion monitoring revealed the presence of alpha-[5,7-(C2H3)2]-tocopherol at a concentration of 0.4 microM, representing 0.8% of the total tocopherol in the plasma sample. This experiment clearly demonstrates that orally administered alpha-tocopheryl quinone is converted in a low overall yield to alpha-tocopherol in humans. The conversion to alpha-tocopherol of that portion of the quinone dose which was actually absorbed into the blood stream may, however, have been fairly efficient.