To investigate the mechanism for the transfer of ribosomal proteins from cytoplasm into nuclei and nucleoli, the uptake of 3H-labelled ribosomal proteins by the isolated rat liver nuclei was investigated in the system containing ATP, GTP, CTP, UTP, and an energy-regenerating system. In the presences of cell sap, the transfer became temperature-dependent. The concentration of ribosomal proteins was also very important for their specific transfer and 10-15 micrograms of ribosomal proteins/ml were suitable in the presence of 10(7) nuclei. Removal of one of the four nucleoside triphosphates from the complete system containing cell sap, especially that of CTP or UTP, resulted in a marked decrease of the uptake. A low concentration of actinomycin D inhibited significantly the transfer of ribosomal proteins, while alpha-amanitin to a less extent. The results indicate that the uptake of ribosomal proteins by liver nuclei is largely dependent on RNA synthesis especially rRNA synthesis. The transfer was enhanced to some extent by ATP alone. Other nucleoside triphosphates were less effective. Non-hydrolyzable ATP analogues, adenosine 5'-[beta, gamma-methylene]triphosphate and adenosine 5'-[alpha, beta-methylene]-triphosphate were only slightly stimulative. Although ATP enhanced the transfer into the extranucleolar fraction to some extent, the maximal transfer not only into nucleoli but also into the extranucleolar fraction was dependent on the rRNA synthesis. Sedimentation analyses of the nucleolar fraction of rat liver nuclei incubated with [3H]GTP or 3H-labelled ribosomal proteins, showed that small but distinct amounts of the both labelled compounds were incorporated into 60S preribosomal particles although most of them were found in ribonucleoprotein particles below 30S which were previously shown to be degraded products containing larger rRNA precursors [Tsurugi, K., Morita, T., and Ogata, K. (1972) Eur. J. Biochem. 25, 117-128].