The results of experiments are described in which a known quantity of sterile deuterated water is injected directly into the pulmonary circulation via the right internal jugular vein of several haemodialysis patients and the deuterium to hydrogen ratio, D/H, in the exhaled lung water was measured using the flowing afterglow mass spectrometry technique. The breath D/H abundance was measured in sequential breath exhalations before and after the injection, providing data that are sufficiently detailed to follow the production and loss rate of D/H in the exhaled breath. Thus, in principle, considering isotope dilution the volume of water in the lungs and pulmonary circulation can be derived. However, it is seen that the maximum abundance that the breath D/H reached was much lower than anticipated by considering the likely volume of blood/water in the pulmonary circulation and so it is deduced that either 'leakage' of the injected deuterated water rapidly occurs from the pulmonary to the systemic circulation and/or isotope exchange of deuterium with hydrogen along the bronchial tree efficiently occurs, thus reducing the D/H in the exhaled breath. This latter phenomenon has important implications to breath analysis in which it is often assumed that so-called alveolar breath concentrations of metabolites reflect blood/systemic levels. Detailed consideration of the breath D/H abundances when the deuterium is equilibrated amongst the total body water, TBW, of the patients, which occurs about 40 min after injection of the deuterated water, allows the TBW of the patients to be estimated.