In the present study we investigated the formation of sulfoxides from N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine (N-Ac-TCVC), N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (N-Ac-1,2-DCVC), and N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine (N-Ac-2,2-DCVC), which are formed in the glutathione dependent bioactivation of tri- and tetrachloroethene. The first aim was to elucidate the enzymes involved in these oxidation reactions. N-Ac-TCVC, N-Ac-1,2-DCVC, and N-Ac-2,2-DCVC are oxidized to the corresponding sulfoxides mainly, if not exclusively, by cytochrome P450 enzymes in liver microsomes of untreated male rats, since no role for the flavin-containing monooxygenase (FMO) could be demonstrated by heat inactivation experiments and by the use of n-octylamine. The sulfoxidation rates were increased when using liver microsomes of phenobarbital and dexamethasone pretreated male rats as well as liver microsomes of dexamethasone pretreated female rats, while no sulfoxide formation was observed in liver microsomes of untreated female rats, suggesting an involvement of cytochrome P450 3A. Also, troleandomycin, a specific chemical inhibitor for cytochrome P450 3A, drastically reduced sulfoxidation rates. The observed rates of sulfoxidation also correlated well with the rates of oxidation of testosterone at the 6-beta-position, a specific marker for P450 3A activity. The second aim of this study was to compare the cytotoxicity of the sulfoxides with the cytotoxicity of the corresponding mercapturic acids in isolated rat renal epithelial cells. Both mercapturic acids and the corresponding sulfoxides were cytotoxic. Cytotoxicity of the mercapturic acids could be blocked by (aminooxy)acetic acid (AOAA), an inhibitor of cysteine conjugate beta-lyase, while the cytotoxicity of the sulfoxides was not influenced by this treatment. Moreover, the sulfoxides were significantly more cytotoxic than the corresponding mercapturic acids at equimolar doses. The results show that mercapturic acids derived from TRI and PER are oxidized to sulfoxides by microsomal monooxygenases from rat liver. The cytotoxicity of the produced sulfoxides could not be reduced by AOAA, consistent with a role of the sulfoxides as direct acting electrophiles (i.e., Michael acceptor substrates).