The changes in organic phosphates of turkey erythrocytes (RBC) have been determined in relation to the changes in oxygen affinity of whole blood during growth of the embryo and poult. On a molar basis, 2,3 diphosphoglyceric acid (2,3-DPG) is the predominant organic phosphate of erythrocytes from turkey embryos during the last week of incubation. However, on the basis of relative % phosphate, 2,3-DPG is the major organic phosphate of the erythrocytes from turkey embryos on day 23 and 25 of incubation only. With the exception of day 23 and 25 incubation, adenosine triphosphate (ATP) represents the major organic phosphate of the erythrocytes of the turkey embryo and poult during the last week of embryonic development and through the first 29 days after hatching. The whole blood P50 during the last week of incubation and the first 8 days after hatching correlates best with the amount of ATP in the erythrocytes. The effects of inositol pentaphosphate on P50 of the whole blood is much more gradual and appears to become of major influence after 2-3 weeks post-hatching.