Tilapias (Oreochromis niloticus) were exposed to copper or chromium in soft water (SW) (similar to 80 mg CaCO3/L, conductivity 1.77 mS/cm) or hard water (HW) (similar to 320 mg CaCO3/L, conductivity 5.80 mS/cm) using 2 exposure protocols (20 mu M for 48 h and 10 mu M for 144 h). Following the exposures, antioxidant enzyme activities [superoxide dismutase (SOD); catalase (CAT); glutathione peroxidase; glutathione reductase; and glutathione S-transferase (GST)] and glutathione (GSH) levels were measured in the liver of fish. SOD and CAT activities of control fish kept in SW were significantly lower than control fish kept in HW. However, the other antioxidant indices (glutathione metabolism) of both control fish were unaffected from water hardness. Acute metal exposures did not alter the glutathione metabolism, whereas SOD activity in SW and CAT activity in both waters changed significantly. In subchronic duration, Cu exposure caused significant decreases in measured parameters, except for GST activity and GSH level. Similarly, GST activity and GSH level were unaffected from Cr exposure. This study showed that SOD and CAT were the most sensitive antioxidant indices, and that glutathione metabolism, in general, was not altered following metal exposures in different waters.