High-resolution gel electrophoresis has been used to detect and quantitate promoter-specific oligonucleotides produced during initiation of transcription in vitro at the lactose operon (lac) UV5 promoter. The resolved products are RNA species of various lengths which correspond to the initial lac mRNA sequence. Quantitation shows that many oligonucleotides can be formed per preinitiation complex, including species as long as hexanucleotide. Synthesis occurs without dissociation of the enzyme, as evidenced by levels of synthesis in the presence of heparin, a selective inhibitor of free RNA polymerase. Thus, RNA polymerase cycles at this promoter in vitro producing oligonucleotides reiteratively. In general, the yield of oligonucleotides decreases when the total concentration of all four substrates is increased or when a missing nucleoside triphosphate substrate is added. Nevertheless, oligonucleotide synthesis persists under all conditions tested. Strikingly, the dinucleotide always represents 50% of the total of all oligonucleotides, even when conditions are manipulated to cause a 100-fold variation in this total. This shows that, after formation of the first phosphodiester bond at the lac UV5 promoter, dissociation of the dinucleotide is as likely as formation of the second phosphodiester bond. As discussed above, after release of a small RNA, RNA polymerase may then begin another RNA chain, which is again subject to premature release. These considerations lead to a model in which RNA polymerase cycles to produce oligonucleotides during initiation of transcription at the lac UV5 promoter in vitro. Production of a long RNA transcript is then essentially an escape from this cycling reaction. The drug rifampicin, which drastically inhibits escape to produce RNA, limits, but dose not prevent, the cycling reaction.