The determination of the total volume of the circulating blood or plasma is assuming increasing importance from both the clinical point of view and that of research (Erlanger,1Rowntree, Brown and Roth,2Stander3). A study of the variation of the blood volume in pregnancy having been projected in this laboratory, it became necessary to investigate the methods for determination of blood volume. This paper deals with the results of the investigation. The extensive literature on the subject discloses two basically different methods in use: the carbon monoxide method of Haldane and Smith4and the dye method of Keith, Rowntree and Geraghty.5When using the former method, one must consider myohemoglobin (Whipple6), the dependence on environmental temperature (Barcroft et al.7), the time lag in saturation of the hemoglobin in the splenic spaces (Barcroft and Barcroft8) and the local differences in cell concentration (Lamson and Nagayama9). An analysis of the data obtained