The effects of a 185-min exposure to 48 degrees C db/33 degrees C wb, on intravascular volume and osmolarity and on intravascular electrolyte, aldosterone, and cortisol concentrations have been studied in five male subjects before and after acclimatization to heat. Changes in the hematocrit and plasma protein concentration indicated that a hemodilution occurred during the first 35 min of the heat exposures, and that this was followed by a hemoconcentration. Although these changes in intravascular volume were not affected by acclimatization, the plasma volume after heat acclimatization was 6.7% greater than before. This increase in plasma volume was associated with an elevation in the ratio [Na]/[K]. However, since plasma osmolarity decreased the intravascular expansion could not be explained in terms of elevated electrolyte levels. Plasma aldosterone and cortisol levels were not affected by heat acclimatization, although both were elevated following exercise in the heat. It is concluded that the adrenal cortex is not an important factor in maintaining a state of heat acclimatization once a salt balance has been achieved.