Biological fate of 14C-labeled butylated hydroxytoluene (14C-BHT) in rat liver was investigated. After a single oral dose of 14C-BHT, the radioactivity was reached a maximum concentration in the liver after 6 hr and in the serum after 6-12 hr. A bulk of radioactivity was found in the supernatant fraction, although the amount of radioactivity and specific radioactivity increased in the microsomal fraction with time. The radioactivity which was incorporated into the microsomal fraction was mainly located in the smooth ER 3-6 hr after the administration. These results suggest that the radioactivity, which was incorporated into the supernatant fraction, migrated to the microsomal fraction.