In this study, freshwater fish Oreochromis niloticus were exposed to acute (3 days) and chronic (30 days) Hg2+ and Ni2+ in differing Ca2+ levels (15, 30 and 90 mg Ca2+/L). In the acute duration 30 mu M Ni2+ and 0.3 mu M Hg2+, in the chronic duration 10 mu M Ni2+ and 0.03 mu M Hg2+ were tested. At the end of the exposures, activities of ATPase (Total-ATPase, Na+/K+-ATPase and Mg2+-ATPase) and AChE were measured in the brain. There were changes upon Hg2+ and Ni2+ effects depending on the Ca2+ concentration though low dependence on Ca2+ exposure alone. Decreased activities were recorded after acute metal exposures even at high Ca2+ concentration. There were variable alterations in ATPase activities whereas AChE activity was mostly altered by Hg2+ exposure. ATPase and AChE activities were changed in both durations due to metal type and Ca2+ concentration differences. However, ATPase responses were more affected. Changes in the activities demonstrated the sensitivity of these vital enzymes towards environmental factors. Therefore, there is a potential using of these enzymes as biomarkers on the basis of multi-biomarker attitude in environmental toxicology studies.