The two CHCl3 activation pathways have been studied in incubations at different oxygenation conditions with hepatic microsomes from control Sprague Dawley (SD) rats or SD rats treated with different cytochrome P450 inducers (acetone, phenobarbital, pyrazole, dexamethasone, and beta-naphthoflavone). The present results provide direct evidence that CHCl3 concentration is critical in determining the role of different cytochrome P450 isoforms (CYP) and the related effects of metabolic inducers. At 0.1 mM CHCl3 concentration, the only major contribution to its oxidative biotransformation in liver microsomes from untreated rats was due to CYP2E1, as shown by metabolic inhibition due to 4-methylpyrazole or by anti-CYP2E1 antibodies. Moreover, animal treatments with acetone and pyrazole increased the production of adducts of phosgene to microsomal phospholipid by about 10-15 times. At 5 mM chloroform, in control rat liver microsomes, CYP2B1/2 was the major participant responsible for chloroform activation, while CYP2E1 and CYP2C11 were also significantly involved. Consistently, at this chloroform concentration, the effect of phenobarbital (CYP2B1/2 inducer) was maximal, producing very high levels of adducts. The reductive pathway was expressed at 5 mM CHCl3 only and was not significantly increased by any of the inducers used. Moreover, it was not inhibited by metyrapone and 4-methylpyrazole or by anti CYP2C11 antibodies. Therefore, it may be concluded that, in the range of chloroform concentrations tested, those CYPs involved in CHCl3 oxidative bioactivation do not participate in CHCl3 reduction. Chloroform oxidative metabolism in PB-microsomes could achieve very high absolute rates, much higher than those in C-microsomes; in contrast, the metabolic rates in AC- and PYR-microsomes remained within the activity levels observable in C-microsomes at high chloroform concentration. Therefore, it can be argued that the CYP2B1/2-mediated induction of CHCl3 activation is the basis for the effect of PB in potentiating chloroform hepatotoxicity. Moreover, processes other than CYP2E1-mediated metabolic induction may be more relevant in the ketones potentiation of chloroform-induced acute toxicity.