The time-course of synthesis of long and short (abortive) transcripts by Escherichia coli RNA polymerase was investigated under single round conditions in vitro. The synthesis of long RNA initiated at the lambda PR (with an altered leader sequence) or at the lacUV5 promoter was completed within five minutes, but abortive transcripts were continuously synthesized for at least 20 minutes. The results indicate the presence of transcription complexes that are capable only of abortive synthesis, and not of productive elongation. Consistently, only one in four of the RNA polymerase molecules that initially associated with the lambda PR promoter synthesized long RNA. The enzyme reisolated from productive complexes synthesized both long and abortive transcripts, behaving just like the original enzyme. This suggests that RNA polymerase is homogeneous with respect to its ability to synthesize the two types of product. Overall, the results indicate that many transcription complexes can be irreversibly trapped in vitro in an abortive cycle.