Six hemoglobin isoforms were identified in the peripheral blood of male Wistar rats by electrophoresis in polyacrylamide gel. Erythrocytes can be separated into 6 fractions by fractional centrifugation; the fractions differ in the level of reticulocytes and cells with fetal hemoglobin. The cells of each fraction contain one light isoform and one heavier isoform, and their combination is specific to each fraction. Changes in the hemoglobin heterogeneity in the whole blood in case of blood loss are associated with changes in physical and chemical properties of circulating erythrocytes, as well as with the formation of various cell populations resulting from activated erythropoiesis.