The products of oxidation of the alpha-tocopherol model compound, 2,2,5,7,8-pentamethyl-6-chromanol (PH) by t-butyl hydroperoxide in chloroform varied with the amount of water present. In the presence of a trace of water, the main products were the spirodimer (PSD) and spirotrimer (PST). As the content of water increased, the main product became 2-(3-hydroxy-3-methylbutyl)-3,5,6-trimethyl-1,4-benzoquinone (PQ). Oxidation of PH in aqueous liposome suspension also produced PQ as the major product. These results suggested that, in aqueous solutions, the major oxidation product of PH would be PQ and of alpha-tocopherol (TH) would be alpha-tocopheryl quinone (TQ). The ease of reduction of PQ and TQ was studied in chemical and biological systems. PQ, TQ, and ubiquinone-10 (UQ) were rapidly reduced to their respective hydroquinones (PQH2, TQH2, and UQH2) at pH 7.3 by NADH plus FAD. Whole blood reduced PQ rapidly at 37 degrees C to PQH2 but did not reduce TQ to TQH2. Human peripheral blood mononuclear cells took up TQ from a bovine serum albumin complex and reduced it to TQH2. Ingestion of TQ (350 mg) by one of us (PSK) resulted in the formation of TQH2 during a 5 h period. These results demonstrate that several biological systems are able to reduce TQ to TQH2 and that it is a reaction that may occur normally in vivo.