Two chemical ionization mass spectrometric methods were developed for direct determination of deuterium in water in the range of 0.0-0.6% 2H2O. One of them utilizes the batch inlet system, methane as the reagent gas, and the peak matching device of a magnetic sector mass spectrometer. The second one utilizes the directly-coupled gas chromatograph of a quadrupole mass spectrometer and computer control for ion selection and data processing. In this method the water itself serves as the reagent gas. The deuterium concentration is calculated from the ratio of ion intensities at m/z 20 (2HH2O+) and m/z 19 (H3O+). We have used these methods to determine total body water in 350 human subjects, which entailed making 900 measurements over a period of four years. Comparisons were made in 200 subjects of our results with those obtained by the creatinine method. No significant differences were found.