2, 6-Di-tert-butyl-4-methylene-2, 5-cyclohexadien-1-one (BHT-quinone methide, BHT-QM) was detected in the bile of rats given butylated hydroxytoluene (BHT). The biliary excretion of BHT-QM during 24 h after administration of BHT, 3, 5-di-tert-butyl-4-hydroxybenzyl alcohol (BHT-alcohol), or 2, 6-di-tert-butyl-4-hydroxy-4-methyl-2, 5-cyclohexadien-1-one (4-hydroxy-BHT) was determined by high-performance liquid chromatography. BHT-alcohol gave about 7 times as much BHT-QM as did BHT, but no BHT-QM was detected after administration of 4-hydroxy-BHT. This result suggests that the transformation of BHT to BHT-QM proceeds mainly through BHT-alcohol. Although 1, 2-bis (3, 5-di-tert-butyl-4-hydroxyphenyl) ethane (BHT-dimer) has been reported as a biliary metabolite of BHT in rats, most of the BHT-dimer seems to be formed artificially by dimerization of BHT-QM during the isolation process.