The equine bone marrow responds to blood loss by increased erythropoiesis, only releasing reticulocytes into the peripheral circulation in severe chronic anemia. We have used morphometric analysis based on electron microscopy of the equine marrow to examine the maturation and release of reticulocytes. Developing red cells in the bone marrows of normal and chronically anemic horses were divided into four stages: early, intermediate, late-stage erythroblasts, and reticulocytes. Morphometric analysis of each stage included volume density of mitochondria per μm3 of cytoplasm, surface area of the outer mitochondrial membrane per unit volume of mitochondria, and the number of ribosomes per unit volume of cytoplasm (total, clustered, single). Matched t tests between normal and anemic animals showed significant differences (P ⩽.001) for volume density of mitochondria and numbers of ribosomes only at the reticulocyte stage. The large reticulocyte produced and released in chronic anemia may be best explained by a skipped mitotic division.