The aim of the present study was to investigate the response of ATPases in tissues of freshwater fish Oreochromis niloticus following exposure to Pb2+ in differing Ca2+ levels. Fish were exposed to Pb2+ in water containing 15 (conductivity of 161.0 +/- 10.7 mu S/cm), 30 (conductivity of 242.0 +/- 9.50 mu S/cm), and 60 (conductivity of 395.2 +/- 40.2 mu S/cm) mg Ca2+/L. Experiments were conducted using two Pb2+ exposure protocols, naming acute (48 h, 20 mu M Pb2+) and chronic (14 d, 10 mu M Pb2+). Following the exposures, Na+/K+-ATPase and Mg2+-ATPase activities in the gill and intestine, and Ca2+-ATPase activity in the muscle were measured. In acute exposure, Ca2+ exposures alone caused significant increases in ATPase activities. Combined exposures of Ca2+ and Pb2+ caused significant increases in ATPase activities at the lowest Ca2+ concentration, but almost all ATPase activities significantly decreased at the highest Ca2+ concentration. In chronic exposures, Ca2+ exposures alone caused a few significant alterations in ATPase activities, clear trend being a decrease in Na+/K+-ATPase activity. In combined exposures, there were only significant increases in ATPase activities at the lowest Ca2+ concentration. Gill Na+/K+-ATPase was found to be the most sensitive enzyme compared to ATPases in the other tissues. Data emphasized that Ca2+ concentration of water is an important parameter in assessment of acute or chronic metal toxicity, especially when ATPases are concerned and should be taken into account in evaluations of biomarkers in metal contaminated waters.