Hexachlorobenzene (HCB) was metabolised by phenobarbital-induced liver microsomes from male rats to pentachlorobenzene, pentachlorophenol, tetrachloro-1,2-benzenediol and tetrachloro-1,4-benzenediol (1:88:2:9). Metabolites were identified and quantified by electron capture g.l.c. Structures were confirmed by selective ion monitoring g.l.c.-m.s. The formation of pentachlorophenol was dependent on the presence of NADPH and O2 and inhibited by CO, SKF 525A and metyrapone. Conversion of HCB to pentachlorophenol was stimulated by pretreatment of rats with phenobarbital (PB) but not by 3-methylcholanthrene (3-MC), or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In contrast, the conversion of pentachlorophenol to tetrachloro-1,4-benzenediol was markedly induced by 3-MC but poorly by PB. HCB, Aroclor 1254 and isosafrole stimulated both hydroxylations. The cytochrome P-450c inhibitor 9-hydroxyellipticine inhibited conversion of pentachlorophenol to tetrachlorobenzenediols by HCB and beta-naphthoflavone induced micromes. In addition to hydroxylation reactions, evidence was obtained for the conjugation of HCB with glutathione catalysed by a microsomal glutathione transferase. Radioactivity from [14C]HCB was bound to microsomal protein during aerobic incubations. Binding was inhibited by GSH and N-acetyl-cysteine. Preliminary studies suggested that the reactive species was derived from tetrachloro-1,4-benzoquinone. No correlation was found between levels of metabolites or covalent binding produced by the two sexes and the marked sex dependent hepatic porphyrogenic and carcinogenic effects of HCB.