Mercury is known to modify enzyme activity through oxidation of thiol groups and respective reverse reactions in vitro and in vivo. However, variations in the activity of carbohydrates, and the significance of this variation after mercury poisoning in different species, has not been established. In the present report, the effects of inorganic mercury on selected hepatic enzymes was studied in the freshwater fish Channa punctatus. Quantitative data clearly showed a dose-response relationship between the amount of mercury retained in the liver and inhibition of enzymes (i.e. alkaline phosphatase, glucose-6-phosphatase, amylase, maltase, lactase, lipase and dehydrogenases). Mechanisms and significance of their modification have also been discussed.