Ten male volunteers were subjected to heat stress in winter and in summer and changes in volume and composition of the body fluids were followed for about 24 hr afterwards.
In summer, sodium and chloride concentration in serum and serum osmolality increased most just after sweating and returned to the control value within a day of heat exposure, while in winter they were highest three hours after sweating, and the effect of heat stress lasted until the following morning. Both in winter and in summer, the circulating blood volume tended to increase even when hemoconcentration was obvious. Plasma protein which was determined in summer experiments showed an increase after heat stress and the importance of plasma protein in maintaining the circulating blood volume during or after sweating and its contribution to so called voluntary dehydration are discussed.