Alterations in ploidy of hepatocyte nuclei were followed in experiments on 190 rats with transplantable tumors during liver regeneration and x-ray irradiation. The presence of the growing transplantable tumor in the animal's body led to alterations in the nuclear structure of hepatocytes, manifesting in the increased ploidy. X-Ray irradiation brought about a rise in hepatocyte ploidy which was more remarkable the higher the dose. Under such conditions, polyploidy of hepatocyte nuclei was more pronounced in tumor-bearing rats than in those without tumors. An abrupt increase in ploidy of hepatocyte nuclei occurred in the regenerating liver of tumor-bearing animals both non-irradiated and, particularly, irradiated ones.