The suspension of rat intact liver cells at different age or with liver regeneration was prepared. The isolated hepatocytes were smeared. The number of nuclei per cell was determined and the DNA content of each single nucleus was measured by a microspectrophotometer at a wavelength of 560nm. The transformation of the diploid population of hepatocytes in newborn animals into a practically exclusive polyploid took place during their postnatal growth. Analysis of rat hepatocytes undergoing liver regeneration following two-thirds partial hepatectomy revealed that there was an increase in tetraploid cells and a decrease in diploid and binuclear ones. The polyploidization appeared to be temporarily accelerated under the stimulus of proliferation.