The amount of DNA in the nuclei of individual megakaryocytes of male Sprague-Dawley rats was measured micro-spectrophotometrically in marrow smears stained by the Feulgen reaction. When the DNA values were arranged in a frequency distribution, the megakaryocytes fell into groups with the modal DNA content of successive groups increasing approximately by a factor of 2. Ploidy of the megakaryocyte groups was determined by comparison with DNA values of diploid cells. Frequencies of 4N, 8N, 16N, and 32N megakaryocytes were, respectively, 1.6%, 10.5%, 71.2%, and 17.1%. The results indicate synchronous division of the nucleus of polyploid megakaryocytes. The relative numbers of cells in ploidy stages may indicate the relative length of time spent in the stages. An hypothesis of megakaryocyte maturation is proposed. The method may prove useful in studies of megakaryocyte maturation and regulation.

ADDENDUM: In a recent microspectrophotometric study, de Leval observed a frequency distribution pattern of DNA of polyploid megakaryocytes of guinea pigs very similar to that of rats described here (de Leval, M., Compt. Rend. Soc. Biol., 1964, vl58, 2198).