Prenatal administration of 3,3', 4,4', 5,5'-hexachlorobiphenyl (HCB) and 3,3',4,4'-tetrachlorobiphenyl (TCB) resulted in decreased plasma thyroid hormones and altered thyroid hormone metabolism in brain homogenates of pregnant Wistar WU rats, their fetusses and their offspring. These effects of HCB and TCB occur at a time when thyroid hormones play a crucial role in neurological development.