Cytophotometry was used to study changes in the nuclear DNA content and size of the nuclei in the regenerating liver of rats exposed to 154.8 mC/kg (600 R) or x-irradiation 10--30 minutes before partial hepatectomy. The irradiated and non-irradiated animals were examined 18, 24, 30, 36, 42, 48 and 60 hours and 3, 7, 14 and 21 days after partial hepatectomy. It has been discovered that during 30 hours after partial hepatectomy the increase in the nuclear DNA content was prevented by irradiation. As compared with the non-irradiated animals, the increase in the DNA content in the nucleus was 12--18 hours late and occurred in a less number of the cells. Enlargement of the nucleus size in the irradiated animals, mainly on the 48th hour after partial hepatectomy, was not accompanied by the respective increment of the DNA content.