The toxicity of 3,3',4,4'-tetrachlorobiphenyl (PCB 77) and 2,3',4,4',5-pentachlorobiphenyl (PCB 118) was investigated in rats following subchronic dietary exposure. Groups of 10 male and 10 female weanling Sprague-Dawley rats were administered PCB 77 in the diet at 0, 10, 100, 1000, or 10,000 ppb for 13 weeks. PCB 118 was administered to males in the diet at 0, 10, 100, 1000, and 10,000 ppb, while the female groups received 0, 2, 20, 200, or 2000 ppb of the congener for 13 weeks. Growth rate and food consumption were not affected by treatment. No clinical signs of toxicity were observed. Increased spleen weight occurred in male rats fed 1000 or 10,000 ppb PCB 77. Male rats receiving 10,000 ppb PCB 118 had increased liver weight and hepatic ethoxyresorufin O-deethylase (EROD) activity. Increased hepatic EROD activity but not liver weight was observed in female rats given the 2000-ppb PCB 118 diet. Increased EROD activity was also noted in male rats given 10,000 ppb and in female groups receiving 1000 or 10,000 ppb PCB 77. Male rats exposed to 10,000 ppb PCB 77 had decreased vitamin A in the liver and lung and elevated levels in the kidney. Liver vitamin A of both 1000- and 10,000-ppb PCB 77 female groups was decreased. PCB 118 had no effects on tissue vitamin A at the levels studied. No hematological changes or serum biochemical changes were seen in any of PCB 118- and PCB 77-treated groups, nor were liver uroporphyrin levels altered. A reduction in dopamine and homovanillinic acid in substantia nigra region of the brain was observed in female rats fed 2000 ppb PCB 118, while 10,000 ppb PCB 77 was associated with an elevation in 3,4-dihydroxyphenylacetic acid in the nucleus accumbens region of male rat brains. Mild to moderate changes were observed in the liver and thyroid of rats given PCB 77 or PCB 118. PCB 118 accumulated in a dose-dependent manner in fat and to a much lesser extent in liver. In contrast, very low levels of PCB 77 residue were found in the tissues examined. Based on the above data it was concluded that NOAEL of PCB 77 is 100 ppb in diet or 8.7 micrograms/kg and that of PCB 118 is 200 ppb in diet or 17 micrograms/kg body wt/day.