Six male subjects exercised for 50 min at 25% (light exercise) and 55% (moderate exercise) of their estimated aerobic capacities in environments of 42 degrees C db, 35 degrees C wb and 30 degrees C db, 24 degrees C wb, respectively. Alterations in the hematocrit, hemoglobin, and plasma protein concentrations, and in the activity of an injected aliquot of isotopically labeled albumin were each used to calculate the percentage change in plasma volume occurring during exercise and recovery. Changes in each measure were consistent with a reduction in plasma volume during exercise and a return to preexercise levels during recovery. There was no significant difference between the measures when exercising in the heat, but during the more severe exercise in the cooler environment disproportional changes in protein, hematocrit, and hemoglobin were observed. Disproportional changes were also seen during the recovery phase, when the hematocrit and hemoglobin concentration indicated a more rapid return of the plasma volume to preexercise levels than did either the plasma protein concentration or albumin activity. During moderate exercise and recovery there was a 1% decrease in red cell volume. It is concluded that exercise accelerates the rate of protein movement from extravascular compartments to the intravascular compartment, leading to elevated plasma protein levels during recovery which favor the return of water to the intravascular space. Hemoglobin concentration is considered to be the most reliable measure of plasma volume change during exercise.