The antioxidant defense system of the organism controls and inhibits all stages of free radical reactions, starting from their initiation and ending to the formation of hydroperoxides and TBA-active products. The main mechanism of control of these reactions is associated with the chain of reversible redox reactions of metal ions, glutathione, ascorbate, tocopherol and other substances, the value of which is especially important for the preservation of long-standing macromolecules of nucleic acids and proteins, some components of membranes. The purpose of the research was to investigate the activity of the enzyme link of the antioxidant defense system of cows organism in experimental fasciolosis. Twelve cows of 4–5 years of age, Black-Spotted breed, were selected for the experiments, from which 2 groups were formed, six animals in each. Animals in control group (C) were clinically healthy. Animals of experimental group (E) were experimentally infected with adolescents. During the research, the rules of compulsory experiments were followed – selection and keeping of analogue animals in groups. The cows' ration was balanced in terms of nutrients and minerals. All animal manipulations were carried out in accordance with the European Convention for the Protection of Vertebrate Animals, which are used for experimental and scientific purposes. According to the results of our research, in cows affected by fasciolosis invasion, catalase activity in their blood decreased by 21.5 %, and superoxide dismutase activity by 30 %. In the research of the glutathione link of the antioxidant defense system, it was found that the activity of both glutathione peroxidase and glutathione reductase is also reduced in experimental fasciolosis. Thus, on the 25th day of the experiment, a decrease in these enzymes by 26.6 and 20.5 % is set. Thus, with the development of fasciolosis in ruminants, the balance in the complex “Antioxidant system – Lipid peroxidation” is disturbed, which in turn leads to complications of the disease. In the future it is planned to investigate the state of the non-enzymatic part of the antioxidant defense system of cows organism with experimental fasciolosis.