Metals are considered as main environmental pollutants and potential threat to aquatic organisms. In this study, it was aimed to investigate the effects of cadmium on glutathione S-transferase activity, glutathione peroxidase activity, glutathione reductase activity, total glutathione and thiobarbituric acid reactive substances contents which are important biomarkers in antioxidant system, were measured in the gill tissue of Oreochromis niloticus. The fish were exposed to 0.1 and 1.0 mg/L Cd for 7 and 21 days. Exposure of Cd caused increases in activities of GST, GPX, GR and level of TBARS compared to control groups at both periods. These increases showed parallelism with the increase in dose and duration. Cd treatment caused a depletion of GSH level in the gill when compared to the control groups at 7 and 21 days. The results demonstrated that Cd exposure could induce important alterations in GST, GPX, GR, GSH and TBARS parameters in the gill tissue which could be used as a target tissue to reflect the oxidative stress caused by metals in fish.