Polybrominated dibenzodioxins and dibenzofurans are of toxicologic interest due to potential occupational and environmental exposure and because of their structural similarity to the highly toxic chlorinated analogues. The excretion and terminal tissue distribution of (3)H-TBDD was studied in male F344 rats for 56 days following single doses of 0, 0.001 or 0.1 micro mol/kg. The major tissue depots of radioactivity in liver, adipose, and skin, and tissue distribution was dose-dependent. At 56 days, liver concentrations in the high dose group were disproportionately increased compared to the low dose group. Liver:adipose concentration ratios were 0.2 and 2.6 at the low and high doses, respectively. Elimination of radioactivity in the feces, the major route of excretion, and urine was also nonlinear with respect to dose. By day 56, feces accounted for approximately 50% of the administered dose at the low dose versus 70% at the high dose. Blood levels of radioactivity declined rapidly with approximately 2% remaining in the blood by 24 hours. Radioactivity levels in the liver peaked by 7 hrs and then gradually declined concomitant with a slow accumulation in adipose tissue. The terminal excretion half-life of radioactivity in adipose was estimated to be approximately 60 days. Liver:adipose concentration ratios declined with time. Thus, the overall disposition of TBDD appears similar to that observed for the chlorinated analogue, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The dose-dependent tissue disposition and excretion kinetics of these compounds suggest important considerations for extrapolations from high to low doses.