A rapid and inexpensive method was developed to determine deuterium enrichment in body fluids. This is achieved by converting water into acetylene. To vacutainer tubes a small amount of calcium carbide is added. The tubes are evacuated and 25 microliters of sample are injected through the stopper. The reaction takes place spontaneously at room temperature in a few seconds. Enrichment at mass 27 compared with mass 26 can be determined by continuous flow isotope ratio mass spectrometry without any interference from the carrier gas helium. A series of D2O samples diluted with increasing amounts of H2O is prepared at the time of measurement of the biological samples and the measured ratios are used to calculate the isotope dilution of the unknown. The relative error of the method is 1.6% when a dose of 25 ml kg-1 is administered to the patient. The method was compared with two different methods in use in other laboratories, by a published method The means of the differences were -0.1 and 0.08 1, respectively, with standard deviations of 0.63 and 3.0.