Increases in salinity affect fish physiology and metal uptake. Thus, freshwater fish Oreochromis niloticus were exposed to Hg2+ in different salinities (0, 1, 10 ppt) for acute (0,3 mu M Hg2+, 3 days) and chronic (0,03 mu M Hg2+, 30 days) exposure protocols. Following the exposures, activities of Na+/K+-ATPase, Mg2+-ATPase and Ca2+-ATPase in the gill and Ca2+-ATPase in muscle of fish were measured and significant (P<0.05) results were remarked. Salinity increase alone did not cause alterations in ATPase activities in the gill and muscle in both acute and chronic exposures. However, there were increases in ATPase activities in gill and muscle tissues of fish exposed to Hg2+ in different salinities both in acute and chronic exposures. In acute exposures, only Mg2+-ATPase activities in the gill increased at 1 and 10 ppt salinities (P<0.05). In chronic exposures, activities of gill Na+/K+-ATPase at 10 ppt, Mg2+-ATPase at 1 and 10 ppt and Ca2+-ATPase at all salinities increased. Similarly, activity of muscle Ca2+-ATPase also increased at 1 and 10 ppt. The most alterations occurred in Mg2+-ATPase activity, followed by Ca2+-ATPase activity and Na+/K+-ATPase activity in tissues of fish exposed to salinity+Hg2+. Overall, chronic exposures affected fish physiology much more that acute ones.