5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB; 75 muM) inhibited only 60-75% of nuclear heterogeneous RNA (hnRNA) synthesis in HeLa cells, while the appearance of labeled cytoplasmic poly(A)-containing messenger RNA (mRNA) was reduced by approximately 95%. DRB-sensitive and resistant transcripts spanned the entire size range of hnRNA molecules. The relative poly(A) content of DRB-resistant hnRNA molecules was the same as that of normal hnRNA. In contrast to the effects of 3'deoxyadenosine, DRB appeared to inhibit the initiation of hnRNA chains, but did not directly interfere with labeling of poly(A). We present an interpretation of these results and propose how the results might be used to gain more information about nuclear RNA metabolism.