Toxic activity of mercury chloride was tested in vivo on fresh water fish Catla catla . Acute toxicity tests were conducted to measure the impact of toxicity on the fishes within a short period at the various concentrations of HgCl₂ (0.1, 0.5, 1, 1.5, 2, 2.5 and 3mg/L). The protein and carbohydrate were estimated using Anthrone by standard methods and enzymes such as Na⁺-K⁺, Mg²⁺ and Ca²⁺ adenoxide triphosphatases were determined caloriemetrically. Depletion of protein was observed at all exposure periods. The biochemical estimation values of carbohydrates in muscle, intestine and brain showed significant values with P<0.05. The activity of Mg ²⁺ adenoxide triphosphatases in muscle, intestine and brain was studied and the maximum depletion was observed at 96h, 72h and 72h, respectively. Regarding activity of Ca ²⁺ adenoxide triphosphatases in muscle, the maximum depletion of this enzyme activity in muscle was observed at all exposure periods, except 96h. In intestine, enzyme activity was gradually decreased in all exposure and an increase was found during 96h. In brain, the enzyme activity was observed at all exposure periods except 96h.