Total superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6) activities in erythrocytes and the glutamic acid-oxalacetic acid-transaminase (GOT, EC 2.6.1.1) and glutamic acid-pyruvic acid-transaminase (GPT, EC 2.6.1.2) activities in the plasma were measured in experimental groups of carps (Cyprinus carpio L.) exposed to cadmium in a concentration of 20 mg Cd/l water under aquarium conditions for 6, 12, 18 and 24 hours and in control fishes. It was shown that the total activity of SOD in the erythrocytes is significantly decreased after 12, 18 and 24 hours of cadmium exposure. Increased activities of CAT (after 24 hours) in the erythrocytes and GOT and GPT in the plasma were found in cadmium-treated fishes. At the same time the concentration of blood haemoglobin and haematocrit values were significantly diminished. These results indicate that cadmium causes oxidative stress and tissue damage in the exposed fishes.