A class of heterogeneous nuclear RNA (hnRNA) molecules contain an internal transcribed poly A sequence of close to 25 uninterrupted AMP residues. HnRNA molecules containing this sequence are separable from those containing the large 3' terminal poly A sequence on the basis of their differential affinity for oligo dT cellulose. The fact that the transcribed small poly A and the 3' terminal poly A are not found in the same hnRNA molecules even though both are present in similar size classes and that the small poly A is absent from cytoplasmic messenger RNA (mRNA) has led us to propose a scheme for mRNA processing in which the 3' end of the small poly A in hnRNA becomes a priming size for the post-transcriptional addiction of the large poly A.