Summary.The total body water of two healthy subjects was measured simultaneously with deutecium oxide and antipyrine. Within two months seven measurements were performed on one of the subjects and eight on the other. The variability of the deuterium spaces, expressed by the coefficient of variation, was 1.8 and 2.0 per cent. The variability of the antipyrine spaces was 4.5 and 5.4 per cent, respectively.The analytical error of the equilibrium concentration of deuterium was estimated to about one per cent, whereas the average analytical error of the zero‐time concentration of antipyrine amounted to about four per cent. The greater variability of the antipyrine space is probably due to this larger error.The accuracy of the measurement of deuterium oxide concentration by the gradient tube method was investigated. The analytical error was found to be ± 0.0017 grams per cent.