1. The distribution and excretion of orally administered 14C-labelled 2,2',4,4'-tetrabromodiphenyl ether (TBDE) have been studied in rat and mouse. 2. TBDE was efficiently absorbed and stored in adipose tissue where high concentrations were observed in both species. 3. In the rat, 86% of the dose was retained after 5 days, while 14% was excreted via the faeces and < 0.5% via the urine. 4. The mouse excreted 20% of the dose via the faeces and 33% via the urine, the latter as a hydrophilic and labile metabolite. 5. Metabolites covalently bound to macromolecules and lipids were noted in tissues and faeces from both species. 6. The major individual compound was parent TBDE in the faeces and tissues although small amounts of five hydroxylated metabolites were indicated by GC-MS. 7. In plasma from both rat and mouse only a few of the hydroxylated metabolites were present, indicating selective retention of these metabolites.