Freshwater fish Oreochromis niloticus were individually acutely exposed to different concentrations (0, 0.1, 0.5, 1.0, and 1.5 g/mL) of Cd2+, Cr6+, and Ag+ for 96 h and 0.05 g/mL concentration of the same metals for different periods (0, 5, 10, 20, and 30 days) chronically. Following each experimental protocol, Na+/K+-ATPase, Mg2+-ATPase, and Ca2+-ATPase activities were measured in the gill, kidney, and muscle of O. niloticus. In vitro experiments were also performed to determine the direct effects of metal ions (0, 0.1, 0.5, 1.0, and 1.5 g/mL) on ATPases. Except Ag+, none of the metals caused fish mortality within 30 days. Silver killed all the fishes within 16 days. Metal exposures generally decreased Na+/K+-ATPase and Ca2+-ATPase activities in the tissues of O. niloticus, although there were some fluctuations in Mg2+-ATPase activity. Ag+ and Cd2+ were found to be more toxic to ATPase activities than Cr6+. It was also observed that metal efficiency was higher in the gill than in the other tissues. Results indicated that the response of ATPases varied depending on metals, exposure types, and tissues. Because ATPases are sensitive to metal toxicity, their activity can give valuable data about fish physiology. Therefore, they may be used as a sensitive biomarker in environmental monitoring in contaminated waters. (c) 2011 Wiley Periodicals, Inc.