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¹ Çukurova University, Faculty of Science and Letters, Department of Biology 01330, Balcali, Adana/ Turkey DOI : 10.4194/1303-2712-v15_2_04 Viewed : 4609 - Downloaded : 4526 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 μS/cm), 30 (conductivity of 242,0±9,50 μS/cm), and 60 (conductivity of 395,2±40,2 μS/cm) mg Ca²⁺/L. Experiments were conducted using two Pb²⁺ exposure protocols, naming acute (48 h, 20 μM Pb²⁺) and chronic (14 d, 10 μM 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. Keywords : Lead, calcium, fish, Oreochromis niloticus, ATPase