The poly(uridylic acid [poly(U)] sequences of 30 to 40 nucleotides found in the heterogeneous nuclear ribonucleic acid (RNA) Of HeLa cells are also present in the poly(adenylic acid) [poly(A)] containing messenger RNA (mRNA) of these cells. Twenty-five percent of the total poly(U) sequences in the cell are found in the cytoplasm after HeLa cells have been labeled for 2.5 h with 32P. This value is close to 40% if only poly(A) containing RNA molecules of the cell are considered. The distribution of poly(U) sequences parallels that of poly(A) sequences in different size classes of cytoplasmic messenger RNA. From the concentrations and lengths of the two sequences it can be estimated that about 20% of the poly(A) containing mRNA molecules could contain one poly(U) sequence. Several lines of evidence have been developed to rule out poly(U) containing heterogeneous nuclear RNA (HnRNA) as the source of the poly(U) sequences in cytoplasm. Poly(U) sequences are also found in RNA molecules of the nucleus and cytoplasm which do not contain poly(A). Although poly(U) sequences are most abundant in nuclear RNA species lacking poly(A), they are well represented in a class of RNA molecules in cytoplasm which lack poly(A), but which otherwise resemble the polysomal mRNA of HeLa cells.