Abstract Tissue hypoxia elicits the production of erythropoietin (Epo), a hormone that stimulates red blood cell production. In young diving mammals, oxygen is stored primarily in the blood, and blood oxygen stores increase significantly during the first weeks of life. In an effort to establish the role of Epo during this period of blood development, this study measured Epo concentration in plasma of 134 harbor seal ( Phoca vitulina) pups and adults. Concurrent measurements of hematocrit (Hct), hemoglobin concentration [Hb], and red blood cell (RBC) counts allowed the evaluation of the effect of Epo on blood oxygen store capacity. Erythropoietin and most blood parameters varied with age. At birth, neonatal [Hb], Hct, and RBC were elevated, possibly due to the rapid expansion of plasma volume associated with growth rates of 0.5 kg/day. In contrast, Epo concentration increased from 6.64 ± 0.83 mU/ml in newborns to 9.53 ± 0.86 mU/ml in early nursing pups. Erythropoietin concentration remained elevated above newborn and adult concentration (5.71 ± 0.79 mU/ml) through weaning, suggesting that Epo was responding to tissue hypoxia brought on by early anemia. Since similar changes in erythropoietin have been documented in terrestrial mammals, it appears that Epo plays a similar role in the blood development of harbor seals.