Isoniazid and rifampin are first line drugs used to prevent and treat tuberculosis. The effects of rifampin co-administration on isoniazid-induced oxidative stress were investigated by the determination of the changes in hepatic metabolizing enzymes and DNA damage. Rats were treated with isoniazid alone (100 mg/kg, i.p.) or co-treated with rifampin (100 mg/kg, i.g.) for 10 or 21 days. Activities of CYP2E1, CYP1A1, CYP3A and glutathione S-transferases (GSTs) were analyzed by specific substrates. DNA oxidative damage by drug treatments was analyzed in precision-cut liver slices by HPLC-MS/MS. Isoniazid significantly increased CYP2E1 activities above control levels after 10 or 21 days treatment (2.25-4.59-fold), indicated by both chlorzoxazone hydroxylase and aniline hydroxylase (p<0.01). Isoniazid treatment decreased activities of cytosolic total GST, alpha GST and mu GST after 21 days (p<0.01). No change in activities of CYP1A1, CYP3A, and CYP3A1 mRNA expression was observed after isoniazid treatment. Rifampin co-administration significantly attenuated isoniazid-induced CYP2E1 levels (p<0.01) and inhibition of mu GST (p<0.01). Rifampin did not increase the formation of DNA adducts induced by isoniazid. These results suggest that rifampin co-administration does not increase isoniazid-induced oxidative stress through hepatic CYP2E1 during short-term treatment in experimental rats.