The intestinal glycosidase activities and kinetic characteristics of carbohydrate hydrolysis change in different ways with mercury accumulation in the muscles of perch from aquatic bodies of European Russia. Amylolytic activity decreases in fish from aquatic bodies with a neutral pH with a growth in Hg content (0.05-0.30 mg per 1 kg fresh weight), whereas sucrase increases. A decrease in the Michaelis constant for car bohydrate hydrolysis reflects an adaptive increase in enzyme-substrate affinity. In fish from acid lakes, the muscle Hg accumulation amounts to 0.18-0.86 mg per 1 kg fresh weight. With a growth in Hg content, the glycosidase activities and enzyme-substrate affinity decrease in most cases, having a negative effect on the assimilation rate of the carbohydrate components of food.