Male, albino rats of the F-344/N strain and mice of the B6C3F1 strain were dosed by gavage, 5 days per week for a toal of 22 doses with 0.03--30 mg/kg of FireMaster FF-1, 0.168-16.8 Mg/kg of 2,4,5,2',4',5'-hexabromobiphenyl (HBB), or corn oil vehicle. A battery of tests was administered at the end of repeated dosing (30 day examination) and 30 days after dosing ceased (60 day test). FF-1 and, to a much lesser extent, HBB decreased body weight and performance on a variety of tests designed to detect neuromuscular dysfunction. Included in these tests were activity in the open field, forelimb grip strength, and muscular reflexes. Visual placement responses were also decreased in some animals, while hypothermia was observed in others. Emotionally, as measured by the number of defecations and urinations in the open field, was not affected by exposure to either compound. At the end of 30 day test, mice were less affected by exposure to these polybrominated biphenyls (PBBs) than rats; rats tended to worsen during the 30 days of no dosing, while mice tended to improve. These experiments indicate that oral dosing with levels of PBBs below those required to produce signs of acute toxicity produced behavioral or neurological toxicity when given repeatedly.