The systemic toxicity of 3,3',4,4',5-pentachlorobiphenyl (PCB 126) following subchronic dietary exposure was investigated in Sprague-Dawley rats. PCB 126 was administered to rats of both sexes at concentrations of 0.1, 1.0, 10, or 100 ppb in their diet for 13 weeks. Another group of rats received a loading dose of 5 micrograms PCB/kg body wt at the start of the feeding period followed by exposure to 10 ppb PCB diet for the same period of time as the other groups. Growth suppression and decreased food consumption were observed in the highest dose groups of both sexes. Increased organ/body weight ratios for the liver occurred in the 10 and 100 ppb groups of both sexes. Rats of both sexes exposed to the highest dose of the PCB also exhibited increased relative kidney, spleen, and brain weights. Hematological and most serum biochemical changes were confined to the 100 ppb groups. These included elevated alkaline phosphatase, bilirubin, cholesterol, and aspartate aminotransferase, and decreased serum glucose, hemoglobin, erythrocytes, hematocrit, and platelets. A dose-dependent increase in liver ethoxyresorufin-O-deethylase activity was observed in rats of both sexes starting at 0.1 ppb. A dose-dependent increase in liver uroporphyrin levels was observed in both sexes and significant changes occurred in the female rats at 1.0 ppb and higher dose groups. Decreased liver vitamin A was observed in the 10 ppb group and higher in both sexes. Kidney vitamin A was elevated in the 100 ppb group. No statistically significant changes were noted in concentrations of brain biogenic amines. PCB 126 residues were 10-fold higher in liver than in fat. Treatment-related histopathological changes were observed in the thymus, thyroid, bone marrow, and liver of rats exposed to the 10 ppb diet, but increased frequency of mild changes was observed in most of these tissues at the 1.0 ppb level. Based on the above data, the no adverse effect level was judged to be 0.1 ppb in the diet or 0.01 micrograms/kg body wt/day.