Two groups of C57BL/6J mice received a single oral dose of 1 nmol/kg 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) alone or in combination with 300 mumol/kg 2,2',4,4',5,5'-hexachlorobiphenyl (HxCB). The disposition of TCDD in liver, fat, skin, spleen, lung and blood was studied at days 3, 7, 13 and 34 after dosage. HxCB cotreatment increased hepatic TCDD levels and, consequently, significant increases of the liver/fat distribution ratio were observed. HxCB cotreatment did not significantly affect TCDD levels in fat or other tissues. The elimination rates of TCDD were not influenced by HxCB cotreatment in any of the tissues. It is concluded that HxCB cotreatment alters the body distribution of TCDD in mice but does not influence the elimination rate of TCDD.