Increased nuclear segmentation was experimentally induced in rat mature granulocytes. Some mature hypersegmented granulocytes were tetraploid, other hypersegmented granulocytes were diploid. The phenomenon of the formation of tetraploid mature granulocytes has not been observed till the present time. Our data suggest that the increase in nuclear segmentation of granulocytes can be stimulated by elevated DNA content, but the process of nuclear lobe formation is not dependent on cellular content of DNA.