Heat stress is one of the main reactions of the body’s response to environmental factors, negatively affecting the welfare of various animal species. An increase in environmental temperature can cause stress, which, in turn, potentially has various negative consequences for animals including disruption of the antioxidant system functioning. Violating the antioxidant-prooxidant balance can lead to an increased free radicals formation in the body, which can damage cells and promote the development of various diseases. The article presents the study results of the artificially simulated heat stress effect on individual indicators of the antioxidant system and lipid peroxidation products in the chickens’ liver. The choice of these animals for this study is due to the fact that the poultry is particularly sensitive to the elevated environmental temperatures and, accordingly, to heat stress. This is caused by the biological peculiarities of birds, in particular, they lack sweat glands, and their body surface is mostly covered with feathers, which, together with the high density of keeping in the industrial poultry farming, often leads to the heat stress. The purpose of this work was to find out the presence or absence of changes in individual indicators of the antioxidant system glutathione link and the lipid peroxidation products content in chickens’ liver. The analysis of indicators such as lipid hydroperoxides (LOOH), TBA-active products, reduced glutathione (GSH), glutathione peroxidase (GSH-Px) and glutathione reductase (GR) will allow us to obtain information about the body’s antioxidant defense system state and the oxidative stress level under heat stress conditions. In this study we used 18 hens of the white Leghorn breed. The research was conducted in the vivarium of the Institute of Animal Biology of the National Academy of Sciences in two stages. During the first stage, chickens were kept at an air temperature of 20°С for 7 days. During the second stage, we created heat stress conditions by increasing the temperature to 35°С 6 hours a day, also for 7 days. It has been found that as the ambient temperature increased, the content of LOOH, GSH, and activity of GSH-Px in the liver of chickens increased (P<0.01), while, on the contrary, the content of TBA-active products and activity of GR decreased (P<0.01). The obtained data prove the heat stress effect on the antioxidant protection glutathione link in chicken liver. Research results can be of practical value for improving the bird keeping conditions in the industrial poultry farming, developing strategies for protection against stress, and improving the conditions to ensure the animals’ welfare and health.