ABSTRACT A ‘switchover’ from synthesis of predominantly embryonic hemoglobin to synthesis of predominantly adult hemoglobin occurs during differentiation of the house sparrow. This ‘switchover’ is demonstrated by a change in electrophoretic pattern generally comparable with that of the chicken, slightly different from those of the duck and man, and, perhaps, dissimilar to that of the red-wing blackbird. Differences in electrophoretic mobility show that 10-day embryos possess one hemoglobin not present in early hatchlings and two hemoglobins not present in adults. Major adult hemoglobin is synthesized by the tenth embryonic day; and minor adult hemoglobin, between the tenth embryonic day and the seventh day after hatching. The major increase in concentration of hemoglobin occurs between 2 and 11 days after hatching, a time when the two predominant embryonic and two predominant adult hemoglobins are being synthesized. The percentage of packed cells (hematocrit) also increases significantly within this period. Similarities of peptide maps for the house sparrow and other species, such as the owl, indicate the presence of α-and β-polypeptide chains in adult house sparrow hemoglobin. The shift to hemoglobins with faster anodic mobilities and the presence of a higher concentration of hemoglobin in adults than in early hatchlings suggest that this synthesis is predominantly in the β-chains.