A correlated autoradiographic and biochemical study of RNA synthesis in the nucleoli of chinese hamster ovary cells has been made. Quantitative analysis of the labeling indicates that the fibrillar ribonucleoprotein (RNP) component is labeled faster than 80S RNP and 45S RNA molecules, but approaches simultaneously a steady-state 3H to 14C ratio or grains/mum2 after 30 min of [3H]uridine incorporation. On the other hand, the 55S RNP, the 36S + 32S RNA, and the granular RNP components have the same kinetic of labeling with [3H]uridine. These results suggest that the fibrillar and granular RNP components of the nucleolus are the ultrastructural substratum of, respectively, the 80S RNP (45S RNA) and 55S RNP (36S + 32S RNA). The possibility that precursors to 80S RNP exist also in the fibrillar region of the nucleolus is strongly suggested by the rapid labeling of the fibrils on the autoradiographs.

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