Sewage treatment plants (STPs) are common sources of a wide variety of chemicals entering into the aquatic environment. We have investigated the effects of STP effluent on oxidative stress parameters in rainbow trout via measurements of several enzymatic antioxidants as well as the molecular antioxidant glutathione (GSH). In addition, we have measured levels of oxidative damage, i.e. protein carbonyls and lipid peroxides. Our findings indicate that STP effluent does contain prooxidants that affect fish. Oxidative damage was a more consistent indicator of exposure to prooxidants than antioxidant enzymes. Protein carbonyls and lipid peroxides were both elevated (1.3- and 1.8-fold increases, respectively) in fish caged at the STP site compared with reference site as was the amount of oxidized GSH (GSSG), which showed a 3-fold increase. Antioxidant enzymes showed no clear-cut response. Additionally, older pollutants that have been present in down stream sediment for decades had no effects on the parameters measured here.