Numerous workers1have established the existence of a distinct lowering of the hemoglobin concentration in the blood of normal pregnant women. The nature of this "physiologic anemia" and of recovery therefrom has remained confused because seldom have intensive studies been made of the blood of normal women month by month from early pregnancy through and after the puerperium. Such studies2show a steady decline in both hemoglobin and red blood cells from early in pregnancy until the end of the second trimester, after which either no further change or a slight rise occurs. However, observations indicate that within ten days of parturition normal women who have had no significant defects of diet or gastric secretion during pregnancy show an abrupt rise of both hemoglobin and red blood cells to essentially the levels held at the beginning of pregnancy. Correlation of these data with known changes of blood volume