The age population distribution of erythrocytes in late pregnancy was evaluated by the volume and density distribution of cells. This approach enables an evaluation of the life span of erythrocytes in the circulation without using radioactive elements. A younger cell population was found in the circulation of women in late pregnancy as compared to nonpregnant women of similar ages, which might be indicative of shortened life span of erythrocytes. This shortened life span might be explained by the fact that erythrocytes made under conditions of increased erythropoiesis and/or increased metabolic rate show acceleration of senescence. This indication of reduced life span of the erythrocytes in late pregnancy may contribute to the understanding of the increased erythropoiesis on the one hand, and reduced hemoglobin concentration on the other hand, which often occurs in late pregnancy.