Data have been presented which indicate that the liquid scintillation spectrometer provides a convenient and accurate method for the assay of tritium in body fluids. By means of arterial disappearance curves and comparative measurements of concentrations in plasma, transudates, and cerebrospinal fluid from 1 to 6 hours after simultaneous injection, the distribution rates of tritiated water were compared to those of deuterium oxide. No significant difference was found in the behavior of these tracers under the conditions of this study. It is concluded, therefore, that THO and D2O are equally reliable for studies on the kinetics of water distribution and for the measurement of total body water.