Changes in the activity and number of nucleoli has been studied in hepatocytes of the same rat before and after partial hepatectomy. In the latter case, the rate of ribosomal RNA (rRNA) synthesis was increased by a factor of 1.7, on the average. The synthesis was evaluated by incorporation of adenosine triphosphate in situ after alpha-amanitine treatment of squashed cells. In 2c, 4c and 8c nuclei, the number of labeled nucleoli was less than in nucleolonemic nucleoli stained with silver nitrate. These values were drawn together during stimulation of rRNA synthesis but the difference was often preserved especially for highly ploid nuclei. The lack of activity in part of nucleolonemic nucleoli was assumed. After stimulation of RNA synthesis, the number of nucleolonemic and satellite nucleoli decreased, while the size of preserved nucleoli was enlarged. The number of nucleoli did not correspond to the nuclear ploidy; their total size was proportional to the gene dosage. The mean level of rRNA synthesis was similar in nuclei of the same ploidy with two, three and four nucleoli. A great variability was detected in the rate of rRNA synthesis as well as in the number of nucleoloneme and satellite nucleoli in hepatocytes of the control and experimental rats. Some difference was observed in the label degree for nucleoli of the same nucleus.