Four groups (n=10) of one year old Labeo rohita were exposed to 96-hr LC50 and sub-lethal (2/3rd, 1/4th and 1/5th) concentrations of MnCl2 for a duration of 30 days in the glass aquaria of 50L water capacity. After an exposure period of 30 days, the activity of peroxidase enzyme in the liver and brain of MnCl2 exposed fish was measured and compared with the control group. Physico-chemical parameters of the test media, viz. pH, dissolved oxygen, carbon dioxide, total ammonia, total hardness, calcium and magnesium were also monitored on a 12 hour basis during the whole experimental duration by following the standard protocol. The results showed that peroxidase activity in the fish increased significantly more than the control fish in both organs after exposure of manganese chloride. The enzyme peroxidase had activity of 0.543±0.004 UmL-1 and 0.274±0.004 UmL-1 in the liver and brain, respectively, in MnCl2 stressed fish. However, in control fish, the enzyme activity was observed as 0.117±0.008 UmL-1 and 0.024±0.005 UmL-1 in the liver and brain, respectively. The regression analyses revealed a significance variable dependence of increase in liver and brain peroxidase activity on the physico-chemical variables of the metal exposed media.