Age plays a determining rôle in the concentration of hemoglobin and erythrocytes in the blood of the albino rat during the period of growth. Sex plays a relatively insignificant role in the concentration of hemoglobin and erythrocytes. The total leucocytes show such variation in individual animals during various stages of growth, that definite conclusions as to the influence of age and sex cannot be made. Pregnancy, on our standard stock diet No. 1, produces an anemia in the albino rat which is more marked on day of delivery of young. With the advance of lactation the anemia disappears. During advanced stages of pregnancy a decided drop in the concentration of serum proteins is apparent. The concentration of hemoglobin and red corpuscles in the lactating rat is greatest near the weaning period of the young when the nurslings begin to partake of the maternal diet.