Madin Reservoir (MR), located in the State of Mexico, is fed mainly by the Rio Tlalnepantla. MR supplies potable water to the municipalities of Naucalpan and Atizapan, and various recreational activities take place in its vicinity, such as sailing and the fishing of diverse species including the common carp Cyprinus carpio. The purpose of this study was to determine the toxic effects of contaminants present in MR water and sediment on C. carpio. Five sampling stations were selected (those considered to have the most problems due to discharges). Water and sediment samples were taken and toxicity studies were performed, including acute toxicity (lethality) and subacute toxicity assays. The biomarkers used in the subacute assays were lipid peroxidation (LPX) and activity of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) in the liver and brain of test organisms. These biomarkers were also evaluated in local carp, i.e. carp with chronic exposure in situ to reservoir contaminants. Results show that contaminants in the water and sediment of the different sampling stations induce oxidative stress, this toxicity being more evident in samples from stations near the entry point of the Rio Tlalnepantla tributary and in local carp. This may be due to high contaminant levels as well as the fact that the physicochemical characteristics of the matrices might favor their bioavailability. Thus, both the water and sediment of this reservoir are contaminated with xenobiotics hazardous to C. carpio, a species consumed by the local human population.