1. The blood volume of the mouse has been measured using 59Fe‐labelled red cells to determine the red cell volume and 131I‐labelled human serum albumin to determine the plasma volume.2. Values for the blood volume of 95·0 ± 1·5, 96·3 ± 2·7 and 84·7 ± 1·2 ml./kg body wt. were found for CSI female, CBA female and CBA male mice respectively.3. A marked discrepancy was observed between the venous (cardiac) haematocrit and the whole body haematocrit.4. The blood volume of the mouse must be determined from the red cell volume and the plasma volume, measured using appropriate labels, and not from the red cell volume or the plasma volume using the venous haematocrit.