Background/Aim. Exertional heat stress is a common problem in military services. The aim of this study was to examine changes in body water and serum concentrations of some electrolites in soldiers during exertional heat stress (EHST), as well as effects of 10-day passive or active acclimation in a climatic chamber. Methods. Forty male soldiers with high aerobic capacity, performed EHST either in cool (20 ºC, 16 ºC WBGT-wet bulb globe temperature), or hot (40 ºC, 25 ºC WBGT) environment, unacclimatized, or after 10 days of passive or active acclimation. The subjects were allowed to drink tap water ad libitum during EHST. Mean skin (Tsk) and tympanic (Tty) temperatures and heart rates (HR) measured physiological strain, while sweat rate (SwR), and serum concentrations of sodium, potassium and osmolality measured changes in water and electrolyte status. Blood samples were collected before and immediately after the EHST. Results. Exertional heat stress in hot conditions induced physiological heat stress (increase in Tty, HR, and SwR), with significant decrease in serum sodium concentration (140.6±1.52 before vs 138.5±1.0 mmol/l after EHST, p < 0.01) and osmolality (280.7±3.8 vs 277.5±2.6 mOsm/kg, p < 0.05) in the unacclimatized group. The acclimated soldiers suffered no such effects of exertional heat stress, despite almost the same degree of heat strain, measured by Tty, HR and SwR. Conclusion. In the trained soldiers, 10-day passive or active acclimation in a climatic chamber can prevent disturbances in water and electrolytic balance, i.e. decrease in serum sodium concentrations and osmolality induced by exertional heat stress.