A method of preparing a nuclear fraction from HeLa cells is described. The fraction does not contain an appreciable amount of the 16 s RNA component of ribosomes and thus appears to be free of cytoplasmic contamination. All of the early precursors to cytoplasmic ribosomal RNA are contained in the nuclear fraction. Sedimentation analysis of the RNA extracted from the nuclear fraction shows a peak in optical density at 45 s and a larger peak that sediments about 30 s. The RNA in the 30 s region is shown to be polydisperse and contains 35 a and 28 s RNA. Previous work has shown that the initial event in the formation of ribosomal RNA is the synthesis of 45 s RNA. It is shown here that the next step is the cleavage of 45 s RNA form 16 s RNA, which emerges immediately into the cytoplasm, and 35 s RNA which remains in the nucleus. The 35 s RNA undergoes a transition to 28 s and then enters the cytoplasm. The cleavage of 45 s to 35 s and 16 s RNA proceeds in the presence of actinomycin but the subsequent behavior of the 35 s RNA is abnormal. Experiments in which radioactive label is chased shows that there is no high molecular weight RNA permanently associated with the nucleus.