Pure hexachlorobiphenyl (HCB) isomers induce a number of changes in parameters of drug metabolism in the chick including changes in cytochrome P-450, liver weight, and p-nitrophenol glucuronyl transferase, but not in testosterone glucuronyl transferase activity. The most active inducers of drug metabolism were 2,3,4,2',3',4'-HCB and 2,4,6,2',4',6'-HCB, while 2,4,5,2',4',5'-HCB produced intermediate effects and 2,3,6,2'.3',6'-HCB was a poor inducer. All HCBs caused uroporphyrin accumulation and increased delta-aminolevulinic acid (ALA) synthetase activity, but only 3,4,5,3',4',5'-HCB, 2,3,4,2',3',4'-HCB, and 2,4,5,2',4',5'-HCB produced gross accumulation of hepatic porphyrins. Tissue HCB concentrations correlated well with hepatic effecs. 2,3,7,8,-Tetrachlorodibenzofuran (TCDF), a contaminant of commercial polychlorinated biphenyl (PCB) mixtures, had no effects on hepatic ALA synthetase activity, porphyrin accumulation, or glucuronyl transferase. TCDF did produce a slight increase in cytochrome P-450, but the increase was smaller than that produced by HCBs.