The morphologic stage of maturation, ploidy level, and cell size of individual megakaryocytes from the marrow of normal rats were determined; the initial labeling index of this population of megakaryocytes 30 minutes after injection of ³HTdr was also determined. The results suggested some inferences about the flow of cells through the various ploidy and differentiation compartments during the maturation of megakaryocytes. Most 8N Type-I megakaryocytes endoreduplicate and become 16N Type-I cells, while a few mature to 8N Type-II megakaryocytes. Most 16N I cells mature to 16N II although a few endoreduplicate to become 32N I cells; the 32N I cells mature only to 32N II. The Type-I morphologic stage contains two subcompartments that we have not distinguished morphologically, an endoreduplicating subcompartment and a post-DNA-synthesis subcompartment. The time parameter of the latter ranges from about nine to 19 hours. The time parameter of the former depends on the number of endoreduplications undergone by an individual megakaryocyte. The size of megakaryocytes increases both with increases in ploidy level and with cell differentiation.