To investigate the influence of [Na+] in sweat on the distribution of body water during dehydration, we studied 10 volunteer subjects who exercised (40% of maximal aerobic power) in the heat [36°C, <30% relative humidity (rh)] for 90-110 min to produce a dehydration of 2.3% body wt (ΔTW). After dehydration, the subjects rested for 1 h in a thermoneutral environment (28°C, <30% rh), after which time the changes in the body fluid compartments were assessed. We measured plasma volume, plasma osmolality, and [Na+], [K+], and [Cl-] in plasma, together with sweat and urine volumes and their ionic concentrations before and after dehydration. The change in the extracellular fluid space (ΔECF) was estimated from chloride distribution and the change in the intracellular fluid space (ΔICF) was calculated by subtracting ΔECF from ΔTW. The decrease in the ICF space was correlated with the increase in plasma osmolality (r = -0.74, P < 0.02). The increase in plasma osmolality was a function of the loss of free water (ΔFW), estimated from the equation ΔFW = ΔTW - (loss of osmotically active substance in sweat and urine)/(control plasma osmolality) (r = -0.79, P < 0.01). Free water loss, which is analogous to 'free water clearance' in renal function, showed a strongly inverse correlation with [Na+] in sweat (r = -0.97, P < 0.001). Fluid movement out of the ICF space attenuated the decrease in the ECF space. We also found a linear relationship between ΔECF and change in plasma volume (r = 0.77, P < 0.01). These results suggest that the maintenance of circulating blood volume during dehydration induced by exercise in the heat is a function of the body's ability to mobilize fluid from the intracellular fluid space, which itself is linked to the sodium concentration in sweat.