This experiment represents the first time that it has been possible to measure a body fluid compartment by direct means during spaceflight. Based on the results observed in the five crewmen in this study, it is concluded that TBW decreases by 3.4% after 1 to 3 days of exposure to microgravity in the Space Shuttle. Some individuals appear to undergo this decrease within 24 hours. This effect may be enhanced by decreased water intake due to nausea associated with SMS.