Hardness is one of the most important factors in water chemistry as it affects fish physiology and metal toxicity. The aim of this study was to investigate osmoregulatory responses in the Nile tilapia Oreochromis niloncus exposed to copper and cadmium (1.0 mu g/mL) in soft water (SW) (hardness 80 mg CaCO3/L and conductivity 1.77 mS/cm) and hard water (HW) (hardness 320 mg CaCO3/L and conductivity 5.80 mS/cm) for 0, 1, 7 and 14 days. Following the exposures, Na+/K+-ATPase activity, ion and Cu levels in the gill, kidney and intestine were measured. There was no fish mortality within 14 days, except Cu exposure in SW which killed all fish between 8 and 12 days. Generally, Na+/K+-ATPase activity was altered by both metal exposures in the gill and kidney as it increased in HW condition, but decreased in SW condition. There were also alterations in Na+/K+-ATPase activity in the intestine as its activity generally decreased. Data, in general, showed that Cd was more effective on Na+/K+-ATPase activity comparing to Cu. However, ion levels altered mainly in the kidney and intestine. Tissue metal accumulation was higher in fish tissues from SW condition comparing to HW condition. Data represented here showed that the effects of metals differed in differing water hardness. This suggests that special attention should be paid to the water chemistry when natural monitoring studies are carried out. This study also suggests that the response of osmoregulation system of fish may be a sensitive indicator under stressful conditions in different natural waters. (C) 2013 Elsevier Inc. All rights reserved.