We report changes in activities of detoxification and antioxidant enzymes as well as lipid peroxidation levels in liver, gills, and brain of Jenynsia multidentata exposed to 1,2- and 1,4-dichlorobenzene (DCB). Fish were captured at an unpolluted area, transported to the laboratory, and acclimated previous to experiments. Exposures were carried out using 1,2-DCB at 0.5, 1, 5, and 10 mg L(-1) and 1,4-DCB at 0.05, 0.1, 1, and 5 mg L(-1). After 24-h exposure, fish were sacrificed and dissected separating liver, gills, and brain of each fish. Organs were used for enzyme extractions, evaluating antioxidant system through the assay of glutathione reductase, guaiacol peroxidase, glutathione peroxidase, catalase as well as detoxification system by measuring glutathione-S-transferase (GST) activity. Additionally, thiobarbituric acid reactive substances (TBARS) method was used to evaluate the peroxidation of lipids. No changes in GST activity were found in liver of fish exposed to DCBs but in gills and brain of exposed fish. The detoxification system was activated at lower concentrations of 1,2-DCB than 1,4-DCB. Antioxidant response is activated in liver at low DCB concentrations, followed by a drop at highest levels. We also found activation of the antioxidant system in gills and brain of exposed fish. On the other hand, we did not observe changes in TBARS concentrations in liver or gills of exposed fish with respect to controls, but in brain of fish exposed to 1,2-DCB (> or =0.5 mg L(-1)) and 1,4-DCB (5 mg L(-1)). Responses of both detoxification and antioxidant systems of J. multidentata suggest that 1,2-DCB is more toxic than 1,4-DCB to this specie. To the extent of our knowledge, this is the first report of oxidative stress induced by DCBs in fish. Our results evidence that the brain is the organ most severely affected by the oxidative stress caused by DCBs.