Objective. To study an effect of mexidol on the performance of rats after light or dark deprivations in the swimming test with a load and to evaluate the state of glycolytic processes under these conditions. Material and methods. The experiment was carried out in the spring on 70 Wistar male rats. Three groups (30 animals) were in natural light conditions. One of them was not affected. The other two groups were subjected to exercise and 30 minutes before it either saline or mexidol was administered intramuscularly. Four other groups (40 animals) for 10 days were under conditions of dark or light deprivation prior to the presentation of physical activity and received either saline or mexidol before the test after deprivation was canceled. A forced swimming test with an additional load, which was presented to animals every day at 10—11 am for five days in a row, was used as a model of physical activity. The level of lactate was determined by colorimetric method. Results and conclusion. Mexidol increased the performance of rats in the swimming test, both under natural lighting conditions and with light desynchronization, contributed to the formation of cross adaptation to physical activity under natural lighting conditions and prolonged this state under conditions of light deprivation, did not change the content of lactate in the blood of rats after exercise in natural lighting conditions and dark deprivation and prevented its rise after light deprivation.