^J.rass carp Ctenopharyngodon idella, fingerlings were exposed to lethal and sublethal concentrations of mercury
and zinc. The changes in haemoglobin (Hb) haematocrit (Ht),osmolality, branchial Na+-K+-ATPase5 acetylcholinesterase
(AchE), sodium (Na*); potassium (K*) and water content were recorded. Blood Hb and Ht showed a general trend of significant decrease in fish exposed to lethal and sublethal concentrations of mercury and zinc. The serum osmolality showed a significant increase on exposure offish to lethal and sublethal concentrations of mercury and zinc. In fishes exposed to lethal concentration of mercury and sublethal concentration of mercury and zinc, the activity of branchial Na+-K+-ATPase was increased, while in case of lethal exposure to zinc, the enzyme activity was reduced. Serum AchE activity decreased in fish exposed to lethal concentration of mercury and sublethal concentration of zinc. On
the other hand, the enzymatic activity was increased in fishes exposed to lethal concentration of zinc and sublethal
concentration of mercuiy. In case of brain, the AchE activity increased in fishes exposed to lethal concentration of mercury and zinc and decreased in fishes exposed to sublethal concentration of mercuiy and zinc. The concentrations of (Na+, K4) in serum, liver and muscle were significantly increased when "fish were exposed to either lethal or subletlial treatments. The water content of liver and muscle increased generally, on exposure to lethal and sublethal concentrations of mercury and zinc.