The binding of radioactive material (s) to macromolecules in the liver and other tissues was investigated in rats, after oral administration of ¹⁴C-BHT. The total radioactivity and the radioactivity bound to macromolecules in the liver reached a maximum in 6 hr and in 6-12 hr, respectively. The rate of decrease of bound radioactivity was slower than that of total radioactivity, and therefore, the binding ratio (bound radioactivity in the term of % of total radioactivity) increased with time. The bound radioactivity in the liver was found in all subcellular fractions, although the radioactivity in the microsomal fraction was higher than that in other fractions. In addition, the pretreatment of animals with phenobarbital further increased the amount of radioactivity bound to the microsomes. The bound radioactivity was also found in other tissues, the binding ratio in the lung and kidneys increased with time and that in the brain and spleen remained at the level of about 50% throughout the experimental period.