Summary.The quantity of ripening substances in young rats is large at birth and afterwards decreases with the fall in the number of reticulocytes in percentage of erythrocytes, and when figure sare reached that accord with those found in mature rats, the rest of the blood picture also resembles that in mature rats.During chronic hemorrhagic anemia a rise in the number of reticulocytes may be seen in rabbits. The reticulocytes do not change in character as regards the time they take to ripen when they are subjected to the same concentration of ripening substances, but on the other hand an increase in the quantity of ripening substances in plasma is found in anemized rabbits. This increase shows that under the conditions here described the reticulocytes in the vascular system must ripen about 20 ‐ 30 % more rapidly than those in normal rabbits.In the two cases investigated an increased erythropoiesis then is followed by an increased amount of reticulocytes ripening substances in the plasm