The distribution of some metals in nuclei and nucleoli of normal and regenerating rat liver was determined. The contents of Ca, Zn, Cu, and Mn in nuclei were less than 3% of those in whole cells. Cr and Ni, which are known carcinogens, were present in cells in much smaller amounts than the metals above, but their contents in nuclei were more than 20% of those in cells. All of these metals were retained in nucleoli; the contents in nucleoli, per milligram of protein, were 3 (Zn) to 18 (Ni) times those in nuclei. Binding of Cr and Ni to nucleoli was more resistant than that of the other metals to treatment with nucleases. The concentrations of Ca, Zn, Cu, and Mn in nuclei and nucleoli changed synchronously with changes in the amounts of nucleic acids and protein in the corresponding fraction of the regenerating liver after partial hepatectomy. The Cr concentration showed no such synchronization.