An acute (96 h-0.1, 0.5, 1.0, 1.5 mu g/ml) and chronic (up to 30 days-0.05 mu g/ml) protocols of Cu and Zn were applied to freshwater fish Oreochromis niloticus to investigate these essential metal effects on the activities of gill, kidney and muscle Na+/K+-ATPase, Mg2+-ATPase and Ca2+-ATPase. In vitro effects of both metals (20 min-0.1, 0.5, 1.0, 1.5 mu g/ml) were also measured to be able to compare both exposure routes. Data showed that ATPase activities, in general, decreased following all the exposure conditions, though there were some increases especially in Mg2+-ATPase activity. Among the enzymes, Na+/K+-ATPase and Ca2+-ATPase appeared to be more sensitive than Mg2+-ATPase to the metals. The data also indicated that effects of Cu on ATPase activity in the tissues of O. niloticus were stronger than the effects of Zn, possibly due to higher toxic effects of Cu. In vivo and in vitro exposures of metals showed similar trends with a few exceptions, especially in the gill. Variability of ATPase activity is determined by tissue type, metal species, concentration and duration. This work showed that even essential metals can alter significantly activities of ATPases in fish and thus suggests using them as a sensitive biomarker in metal contaminated waters.