Although the morphology of the hematopoietic and circulating blood cells of the rat differs slightly from that of human blood cells, the basophil is the only human blood cell for which the rat does not have a readily recognizable counterpart. The morphological classification of the rat's marrow and peripheral blood cells as based on our experience is described with reference to the differing observations of previous investigators and with comparison to the human hematopoietic system. Examination of the bone marrow and peripheral blood demonstrated that control rats injected i.v. with 1% normal rat serum in sterile saline exhibit a moderate transient lymphopenia as the only significant hematologic fluctuation induced by the stresses of injection, general anesthesia, and tail bleeding. Morphologic quantitation of marrow and peripheral blood cells complements the insights obtained by the culture of marrow cells. An appreciation of the morphology of the rat's blood cells and of the reproducible and quantitative nature of marrow and peripheral blood smear examinations may lead to more in vivo investigations of hematopoiesis and leukocyte trafficking.