This study was designed to evaluate the effects of different concentrations of copper (Cu) and copper+calcium (Cu+Ca) mixtures in the liver, kidney, and brain tissues of Oreochromis niloticus. The fish were exposed to 0.1, 0.5, 1.0 mg/L Cu and 0.1 mg/L Cu+1.0 mg/L Ca, 0.5 mg/L Cu+5.0 mg/L Ca, 1.0 mg/L Cu+10.0 mg/L Ca mixtures for 7, 14, and 28 days to observe Cu accumulation, total glutathione (tGSH) level and glutathione S-transferase (GST) activity. The alterations in these parameters were determined. Cu accumulation increased in the tissues with increasing concentrations of Cu in the exposure medium and with increasing duration of exposure. Cu levels in the tissues, exposed to Cu + Ca mixtures were significantly lower than exposed to the single Cu. GST activity increased significantly in the liver on the contrary of decreasing in the kidney and brain under Cu exposure. Cu treatment caused a depletion of GSH level in the brain, while increasing in the kidney and liver. In conclusion, Ca exposure seemed to reduce the severity of Cu-induced toxicity in the tissues of O. niloticus.