The aim of the research was to study the effect of photoperiodism disturbances on platelet aggregation activity. Material and methods. The experiment was conducted in two stages in 5 groups of male rats: the control group and four experimental groups. At the first stage, the animals were exposed to 24 h/day continuous light (24L:0D) for 10 and 21 days. In the second stage, animals after 10 and 21 days of round-the-clock lighting were returned to natural lighting for a period of 21 days to study the reversibility of changes. Animals were withdrawn from the experiment by intramuscular injection based on body weight of drugs for anesthesia (telazol, xylanite). Blood sampling was performed by puncture of the right heart. The functional activity of platelets was determined no later than 3 hours after the collection of blood. Results and discussion. The results of the experiment indicate a direct effect of light desynchronosis on the increase in platelet aggregation ability, while it was noted that the degree of increase in aggregation activity and reversibility of changes directly depend on the duration of round-the-clock lighting. So, a 10-day stay of animals in conditions of abnormal lighting leads to an increase in aggregation indices, which remain at a high level even when animals return to normal conditions. In animals that were under round-the-clock illumination for 21 days, a more pronounced increase in platelet aggregation values relative to animals on day 10 was noted. In this regard, it can be argued that circadian rhythm disturbances provoke the development of microvasculature disorders.