We have used the galK gene, minus its promoter, to quantitate transcription of the orfE-pyrE operon of Escherichia coli in front of and after the intercistronic attenuator. Expression of the hybrid genes was studied in a bacterium with mutations that permit changes in the UTP and GTP pools during exponential growth. It was found that the greater part of pyrE gene regulation by the nucleotides takes place at the intercistronic attenuator and that promoter control contributes only little, ca. twofold. When pools of both UTP and GTP were high only 5%–6% of the mRNA chains were continued into the pyrE gene. However, when the UTP pool was reduced (from 1.3 to 0.2 mol/g dry weight) nearly 100% of transcription passed the attenuator. Likewise, a reduction in the GTP pool (from 3.2 to 0.8 mol/g dry weight) resulted in 25%–30% escape of attenuation. Regulation by attenuation disappeared when a premature stop-codon was introduced near the end of orfE such that translational coupling to transcription was prevented in the attenuator area. Therefore, we attribute the modulation of attenuation to nucleotide-induced variations in the kinetics of mRNA chain elongation. In support for this it was found that an RNA polymerase mutant with reduced RNA chain growth rate transcribed past the pyrE attenuator at a high frequency in the presence of a high UTP pool, but only when coupling of translation to transcription was allowed at the end of orfE.