Although a lot of attention is focused on problems of energy balance and nutrition in cold environment, water balance has received less attention. In a cold environment the water balance might be disturbed because the need of water could be increased, the use of water could be decreased and the redistribution of blood could change water volume in circulation. In dehydrated subjects, exercising at -15 degrees C at submaximal work level, both oxygen uptake and heart rate were significantly higher during water deprivation while the anaerobic threshold was lower. The time before reaching exhaustion was also shortened. However, maximal oxygen uptake and maximal muscle strength were not affected. The results suggest lower efficiency, higher physical strain and earlier exhaustion of dehydrated subjects in cold. After repletion of 1.8% body weight loss by an equal amount of fluid (5% sucrose solution) the oxygen uptake was significantly decreased at a submaximal work level at -20 degrees C, suggesting an improved mechanical efficiency and decreased physical strain. Although physical performance could be restored by rehydration, a rapid rehydration is not recommended because of increased diuresis, increased blood pressure and vigorous shivering stimulated by cold fluids. Instead, a continuous maintenance of water balance is recommended, with a fluid temperature above 25-30 degrees C and with a carbohydrate content below 7%.