Halogenated dibenzofurans are found worldwide as environmental pollutants. Structurally related to other halogenated aromatic xenobiotics, their toxicity and disposition seem to vary with the degree and position of halogenation. This work has established that 2,3,7,8-tetrachlorodibenzofuran (TCDF), an extremely toxic isomer, is excreted only after metabolism and toxicity is inversely related to metabolic capability. Thus, rats and mice, which readily metabolize TCDF, are relatively insensitive to its toxic action. Monkeys, which are more sensitive than rats and mice, metabolize TCDF at a slower rate. Guinea pigs, which are exquisitely sensitive to the toxic action of TCDF, fail to metabolize this compound at all. In fact, the lack of clearance results in accumulation upon low-dose chronic exposure. The concept of a threshold body burden for toxicity is currently being tested. The absorption, distribution, metabolism and excretion of two different hexachlorodibenzofurans (HCDFs) is being investigated in rats. The nature of the metabolites produced from these HCDFs and TCDFs is being examined.