The study dealt with the effect of high temperatures (37 and 43 °C) on the formation of superoxide anion radical, hydrogen peroxide, superoxide dismutase (SOD) activity and the accumulation of malondialdehyde (MDA) in wheat leaves. The exposure of plants to the said temperatures was found to promote superoxide anion radical production in leaf cells. This process developed more actively under the effect of the 37 °C temperature.SOD activity in the leaves of seedlings exposed to this temperature increased on the 2nd and 3rd days, while and at 43 °C the rise started as soon as after 15 minutes of exposure and continued until the end of the experiment. Also, exposure to 43 °C promoted hydrogen peroxide formation in wheat leaves, and the rise was especially pronounced after two and three days of the experiment. Plants exposed to 43 °C also exhibited a high content of MDA, which was significantly higher than in the leaves of seedlings exposed to 37 °C. Having compared changes in the formation of ROS, SOD activity and MDA content with the development of resistance in the seedlings at 37 and 43 °C temperatures, we can conclude that in the first case, reactive oxygen species (ROS) can be considered not only as a damaging factor for biological structures, but also as mediators in the process of activating adaptive mechanisms. In the second case (43 °C), the intensive formation of ROS indicates the development of destructive processes, leading to plant damage and death.