Until recently, pentabromodiphenyl (PentaBDE) and decabromodiphenyl (DecaBDE) ethers were commonly used as flame retardants in a wide array of products, mostly in the production of plastics utilized in the electric, electronic and textile industries. The aim of this study was to compare the toxicity of PentaBDE and DecaBDE after their repeated (7-28 days) intragastric administration to rats. The compounds were given at doses of 2, 8, 40 or 200 mg/kg/day (PentaBDE) and 10, 100 or 1,000 mg/kg/day (DecaBDE). The repeated administration of PentaBDE disturbed redox homeostasis, which was manifested by lower total antioxidant status and increased activity of glutathione reductase in serum and higher concentrations of glutathione reduced and malondialdehyde in the liver. The occurrence of these effects was not observed after DecaBDE administration. The results of histopathological examination showed fatty degeneration after administration of the highest dose of PentaBDE. The repeated administration of PentaBDE also caused the increase in relative liver mass, dose-dependent increase in the activity of CYP 1A (EROD) and CYP 2B (PROD), 7-12- and 2-8-fold, respectively, as well as enhanced level of CYP 1A1 (5-30-fold) and CYP 4A (2-4.5-fold). The administration of DecaBDE induced much less pronounced changes: a maximum 2.8-fold increase in the activity of CYP 1A, a twofold increase in CYP 2B, and no alterations in other parameters under study. Contrary to DecaBDE, PentaBDE disturbed redox homeostasis, and induced liver microsomal enzymes. Fatty degeneration in liver caused by this compound was also found.