The relationship between nuclear and cytoplasmic ribonucleic acid (RNA) was examined in HeLa cells growing exponentially in suspension culture. The cells were exposed to radioactive precursors of RNA and then transferred to non-radioactive medium. Observations were made directly after transfer to non-radioactive medium, when the concentration of radioactive precursors in the intracellular pool was still high, and 12 h later when the concentration of radioactive precursors in the pool was much reduced. The labelled nuclear RNA was separated from the labelled cytoplasmic RNA and the specific activity of the individual bases in both determined. It could be shown (1) that the labelled cytoplasmic RNA was synthesized from acid-soluble precursors by pathways which did not necessarily implicate the labelled nuclear RNA, and (2) that, within the limits of measurement, the labelled nuclear RNA was not transferred to the cytoplasm in a stable form but was rapidly broken down within the cell. The behaviour of the nuclear RNA in multiplying cells is thus seen to be essentially similar to that previously described in non-multiplying cells. In the light of the present findings the available evidence suggests that at least some of the nuclear RNA will prove to be labile in many, if not all, animal and plant cells.