The ability to measure total body water accurately, non-invasively and rapidly with results that are immediately available would represent an important advance in body composition research. Flowing afterglow mass spectrometry (FA-MS) has been developed to enable immediate measurement of deuterium content in breath water from single exhalations, which when combined with oral D₂O loading enables measurement of total body water. We report here its first use in normal subjects, demonstrating the dispersal kinetics of deuterium in the body by monitoring deuterium in breath water following ingestion. Ten studies were performed in six normal subjects. After obtaining baseline samples, each subject ingested 0.3 g kg⁻¹ of 99.9% pure D₂O. Subsequently, breath samples were obtained every 3–5 minutes until equilibration had occurred. Three distinct phases in breath deuterium content were observed: an initial immediate peak due to HDO remaining in the oral cavity, a secondary peak reflecting gastrointestinal absorption and finally equilibration with the body water. The incremental increase in breath deuterium abundance between baseline and equilibration was used to calculate the total body water. Mathematical fitting of this final equilibration phase demonstrated that the measured deuterium abundance was within 0.2% of the ideal (asymptotic) value within 2 hours in all cases. We conclude that FA-MS is a powerful new method that, when combined with oral D₂O loading, enables measurement of the dispersal kinetics of HDO and the calculation of total body water within 2 hours.