ABSTRACT:We have previously suggested that the decrease in the levels of serum total thyroxine (T 4 ) and free T 4 by a single administration to rats of Kanechlor-500 (KC500) at a dose of 100 mg/kg is not necessarily dependent on the increase in hepatic T 4 -UDP-glucuronosyltransferase (UDP-GT). In the present study, we determined whether or not a consecutive treatment with KC500 at a relatively low dose (10 mg/kg i.p., once daily for 10 days) results in a decrease in the level of serum total T 4 and further investigated an exact mechanism for the KC500-induced decrease in the T 4 . At 4 days after final treatment with KC500, the serum total T 4 and free T 4 levels were markedly decreased in both Wistar and UGT1A-deficient Wistar (Gunn) rats, whereas significant increases in hepatic T 4 -UDP-GT activity were observed in