The effect of 2,3′,4,4′,5′-pentachlorobiphenyl (CB118) on serum total thyroxine (T₄) level was comparatively examined between C57BL/6 and DBA/2 mice, which are sensitive and insensitive, respectively, to aryl hydrocarbon receptor-mediated biological changes. After 5 d of CB118 administration (50 mg/kg, intraperitoneally (i.p.)), the serum total T₄ levels in both strains of mice were markedly decreased. However, significant decreases in serum thyroid-stimulating hormone levels were observed in DBA/2 mice, but not in C57BL/6 mice. In contrast, significant increases in the level and activity of hepatic T₄-uridine 5′-diphosphate (UDP)-glucuronosyltransferase by CB118 treatment were observed only in C57BL/6 mice. Likewise, significant increases in the amounts of biliary [¹²⁵I]T₄ and [¹²⁵I]T₄-glucuronide after injection of [¹²⁵I]T₄ were observed only in the CB118-pretreated C57BL/6 mice. The CB118-mediated changes in the levels of [¹²⁵I]T₄ bound to transthyretin (TTR), albumin, and thyroxine binding globulin (TBG) were also observed in C57BL/6 mice, but not in DBA/2 mice. Despite such strain differences, significant increases in the liver-selective accumulation of [¹²⁵I]T₄ by CB118-pretreatment was observed in both C57BL/6 and DBA/2 mice. The present findings indicate that CB118-mediated decreases in levels of serum T₄ in C57BL/6 and DBA/2 mice occur mainly through enhanced accumulation of hepatic T₄.