The message-sequence content of pulse-labeled poly(A)+ HeLa [human cervical carcinoma] heterogeneous nuclear RNA (hnRNA) was examined by hybridizations to an excess of message c[complementary]DNA. Control experiments showed that the message cDNA accurately reflected the sequence distribution of the complex mixture of poly(A)+ messages present in the HeLa cytoplasm. Pulse-labeled poly(A)+ molecules in both the lamina-associated and hnRNA fractions contain message sequences, and approximately 65% of the poly(A)-adjacent hnRNA sequences are homologous to the 3'' ends of mRNA. The majority of the pulse-labeled hnRNA molecules contain abundant message sequences. Some pulse-labeled polyadenylated message sequences are still synthesized in the presence of the adenosine analog 5,6-dichloro-.beta.-D-ribofuranosylbenzimidazole under conditions where little or no new cytoplasmic mRNA is produced.