Relationships between red ceil volume ⁵¹Cr-cell), total blood volume (red cell volume divided by hematocrit ratio), and extracellular fluid volume (SCN distribution space) and body weight (ranging between 73 and 384 grams) or lean body mass were studied in 59 nembutalized rats. Lean body mass was determined by means of underwater weighing method on rats clipped and eviscerated. There were positive correlations between body weight or lean body mass and the absolute values (in milliliters) of body fluid volumes. Body fluid volumes expressed on the body weight or lean body mass basis, however, showed negative correlations between body weight (grams) or lean body weight (grams) with one exception. Red cell volume expressed as % lean body mass showed a positive correlation with lean body mass. The other results are summarized as follows: 1. Body density of rats was 1.0561 (range: 1.0123 ~ 1.0781) and 19.8% body weight of total body fat was obtained. The mean value of lean body mass was 80.2% body weight 2. The correlation between body weight and lean body mass was high, namely, coefficient of correlation was r=.99. 3. The correlation between the absolute value of red cell volume (ml) and body weight showed a high correlation, namely, r= 92 and between the lean body mass coefficient of correlation was r=.93. On a weight basis, red cell volume was 2.67 ml/100 gm body weight or 3.48 ml/100 gm lean body mass. The coefficient of correlation between body weight (grams) and red cell volume (% body weight) was r=-. 30. The coefficient of correlation between lean body mass (grams) and red cell volume (% lean body mass) was r=. 50. Thus, the following regression equation was obtained. Red cell volume (% lean body mass)=. 00243 Lean body mass (gm)+3. 12. 4. Total blood volume was 6.06% body weight or 7.83% lean body mass. The correlation between these blood volume values and body weight or lean body mass were negative, namely, r= -.43 and r=-.42 respectively. 5. Extracellular volume (SCN space) was 30.0% body weight or 37.2% lean body mass. These percentage values showed negative correlations between body weight or lean body mass and coefficients of correlation were r=-.40 and r=-.54 respectively. 6. The rate of increase in body weight or lean body mass is accompanied by a smaller rate of increase in blood volume and extracellular fluid volume. The rate of increase in red ceil volume paralled that of lean body mass.