We determined erythrocyte 2,3-diphosphoglycerate (2,3- DPG), Po250%, whole blood hemoglobin concentration, and available O2from the 12th to the 30th d after birth in two matched groups of young rabbits. One group received iron parenterally on the 12th, 15th, and 18th d and the other received no iron supplement. In the untreated group there was a marked fall in hemoglobin concentration from the 12th to the 22nd d and thereafter a marked increase to the initial level on the 30th d whereas the iron-treated animals showed a marked rise in hemoglobin concentration from the 12th to the 22nd d, and a subsequent, slight decline from the 22nd to the 30th d. The average values of Po250% and 2,3-DPG, and the changes in Po250% and 2,3-DPG were virtually identical for both groups. During the first period (12-22 d) there was a marked rise in both 2,3-DPG and Po250% whereas in the second period (22-30 d) there was a somewhat smaller rise in 2,3-DPG and only a slight tendency toward a further rise in Po250%. In the untreated animals “available O2,” reflecting the O2delivery capacity of the blood, remained unchanged during the period of fall in hemoglobin concentration and showed a rise during the second period. In the iron-treated animals “available O2” rose markedly during the first period, with iron-treatment, and remained unchanged during the second period. We conclude that the marked postnatal rise in 2,3-DPG and Po250% in the rabbit seems to be independent of the changes in the hemoglobin concentration.