Introduction. As a kind of sport, winter swimming differs from pool- or open-water swimming due to the extreme cold stress it places on the body. Swims in cold water with facial immersion trigger a diving reflex, which may be accompanied by the development of cardiac arrhythmias under low-temperature conditions. We believe that specific criteria should be developed for permitting individuals to engage in cold-water swimming. Objective. Study of the characteristics of changes in the state of the cardiovascular and nervous systems, autonomic regulation, and metabolic parameters in winter swimmers at water temperatures from +0.5 °C to +2.0 °C both over various standard distances and during a 10-min swim of 450–550 m. Materials and methods. Under competitive conditions and during training camps (at water temperatures from +0.5 °C to +1.5 °C and air temperatures from –15 °C to –18 °C), 24 winter swimmers were examined. These were 9 women aged 23–55 (mean age 39.1 ± 2.7 years); 3 men aged 71 ± 3 years; and 12 men aged 35–60 (mean age 43 ± 1.8 years), all of whom had received medical clearance for participation. The participants' condition was analyzed before and after swims at distances of 25 m backstroke, 25 m and 200 m front crawl (freestyle), as well as after a 10-min swim of 400–450 m. The functional state of the body was assessed using 12-lead electrocardiography, blood pressure measurement, determination of capillary blood glucose concentration, and assessment of simple sensorimotor reaction to a light stimulus. Statistical analysis to identify differences between parameters was performed using the GraphPad Prism 8 software package for Windows 10. Results. A statistically significant increase in the mean capillary blood glucose levels was observed after swims of 25 m (p < 0.05), 200 m (p < 0.01), and 400 m compared to the baseline. An increase in the simple sensorimotor reaction time after 10-min swims was noted; thus, the baseline of 263 ± 10 ms vs. 328 ± 21 ms after the swim (p < 0.01). According to ECG data, at baseline, 67% of the examined individuals showed a widened P wave > 0.11 ms. After short-distance swims of 25 m, P wave duration exceeded the upper limit of normal in 85% of the examined. Compared to backstroke swims, swims with facial immersion in water were accompanied by a significantly more pronounced widening of the P wave (p < 0.05), slowing of atrial conduction PQ (p < 0.01), and a more pronounced increase in the QTc interval (p < 0.05). The QTc value progressively increased in accordance with the duration of the swims. During a 10-min swim, 50% of the examined individuals showed a QTc > 500 ms. Conclusions. Low water temperature is a factor that, even in cold-adapted athletes, provokes stress accompanied by an increase in blood glucose. During swims in cold water with facial immersion, the activated diving reflex triggers cardiac arrhythmias. During long-distance swims, conduction in the nervous system slows down, manifested in an increased simple sensorimotor reaction time and slowed myocardial conduction, reaching pathological values. In this regard, we believe that an additional criterion for medical clearance — assessment of the response of the cardiovascular system to cold-water immersion — should be developed and implemented for beginners wishing to engage in cold-water swimming, in order to avoid the risk of a pathological cardiovascular reaction.