The nucleoside analog 5, 6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB) at 75 to 150 micromolar concentrations inhibits the synthesis of nuclear heterogeneous RNA (hnRNA) in HeLa cells by 60 to 70 percent. The sedimentation profile of hnRNA labeled with (3H)uridine for 45 seconds after brief treatment (45, 90, or 180 seconds) with DRB showed a progressive decrease in the labeling of shorter hnRNA molecules relative to longer molecules. Prior exposure of the cells to actinomycin D, an inhibitor of RNA chain elongation, did not alter the sedimentation profile of hnRNA. These results suggest that DRB preferentially inhibits the initiation of hnRNA chains so that after exposure to DRB for a brief period the longer nascent chains still remain to be finished and thus incorporate a greater share of the pulse label. By progressively increasing the time of exposure to DRB, and measuring the rate of increase in the average size of the labeled, nascent RNA, it was estimated that the chains were growing at rates between 50 and 100 nucleotides per second.