The methods of choice for determinations of a number of key physiological parameters, including body composition, water turnover rates, total energy expenditure (by the doubly labeled water method), and maternal-to-infant human milk transfer, require dosing of individuals with stable isotopically labeled water with subsequent high precision determination of isotope ratios, deuterium/hydrogen and 18O/16O, in biological fluids. Mass spectrometry is generally used for analysis of water isotopes in human studies. However, the excessive time required for workup and instrumental analysis by conventional means severely limits the number of samples which may be analyzed from any one study, thereby severely limiting the range of applicability of these techniques. The proposed work aims to evaluate a novel instrumental strategy to water isotope ratio determinations. This approach is based on two changes to the conventional procedures: 1) direct introduction of liquid water to the ion source of the mass spectrometer to eliminate memory effects, and 2) detection of negative ions to circumvent the problem of unresolvable interferences in the positive ion mass spectrum of water. Direct liquid introduction to the ion source coupled with high speed cryopumping of water is expected to eliminate memory effects due to gas inlets systems, which are the major source of memory observed during direct water analysis. Negative ion mass spectra in the mass range m/e=16-19 will eliminate two ionic species, H2O and H3O which are stable positive ions but are not observed as negative ions. Analysis of the remaining species in this mass range, OH- and O-, will permit direct quantification of both isotopes simultaneously. The major advantages of this continuous flow method are 1) major simplification and time savings for preparation procedures which will require, at most, cryogenic purification, and 2) a reduction in analysis time from the current 40 minutes to about 1 minute for both isotopes. These advantages will expand tremendously the applicability of techniques requiring determination of water isotopes.