STUDIES on the influence of the pituitary on hematopiesis have resulted in general agreement that following hypophysectomy there is a 30% anemia as judged by a reduction in the hemoglobin concentration, red blood cell count, and hematocrit (reviewed by Arvy, Gabe and Stutinsky, 1948). The determination of the concentration of red blood cells and hemoglobin in the peripheral blood gives no real indication of the total circulating red cell volume because of the possibility of the influence of changes in the blood volume. This study was designed to determine the changes in the total circulating red cell volume following hypophysectomy in the rat. methods Blood volume determinations Avere done by the red cell dilution method using P32-labeled erythrocytes (Berlin et al., 1949). Female rats of the Long-Evans strain were hypophysectomized at 26–28 days and the blood volume determinations were done on groups of animals prior to hypophysectomy and at various intervals up to 258 days following the operation (Table 1 and Table 2 and Figure 1).