The metabolism of the suspected carcinogen trichloroethylene (TCE) was studied in in vitro systems involving purified rat liver cytochrome P-450; rat, human, and mouse liver microsomes; rat lung microsomes; and isolated rat and mouse hepatocytes. The studies support the view that metabolism of TCE proceeds through formation of a complex with oxygenated cytochrome P-450 which, by rearrangement, can lead to: (a) suicidal heme destruction; (b) formation of chloral, which can be either reduced to trichloroethanol and conjugated to form a glucuronide or oxidized to trichloroacetic acid; (c) formation of TCE oxide, which decomposes to carbon monoxide and glyoxylate; and (d) metabolites which bind irreversibly to protein, DNA, and RNA. Studies with microsomes and reconstituted enzyme systems suggest that the contributions of the four major pathways described above vary depending upon the isozymes of cytochrome P-450 involved and that these pathways cannot be strictly correlated. Conjugation of products with glutathione does not appear to play a major role in TCE metabolism. Treatment of rats and mice with phenobarbital resulted in a number of alterations in metabolism which were more pronounced in the isolated hepatocyte system than in fortified microsomal incubations. In several cases where hepatocytes were used, the bulk of the metabolites which became irreversibly bound to DNA and protein could be trapped outside of the cells by including such macromolecules in the system, implying that metabolites which bind irreversibly must possess a reasonable degree of chemical stability. The results suggest that TCE oxide is not the TCE metabolite responsible for irreversible binding to protein and DNA. The levels of protein adducts and particularly DNA adducts formed were substantially higher in isolated C57BL/6 x C3H F1 mouse hepatocytes than in isolated Osborne-Mendel rat hepatocytes, and these results may help to explain species differences previously reported in carcinogen bioassays.