Hepatocyte nuclei in several species of vertebrates were examined, to establish the frequency of polyploidy and related parameters along evolutionary lines. Nuclei were compared in terms of volume, DNA content, ploidy ranges and internal organization. Several trends emerged. When present, heterochromatin occupied 20--25% of nuclear profile areas, irrespective of nuclear volume and poloidy; the volume of heterochromatin, however, increased in direct proportion to ploidy level. Regardless of internal organization, ploidy and species, a direct correlation emerged between the volumes of nuclei and their absolute DNA content. Results are discussed in terms of structural and genic DNA.