Modulation of transcriptional elongation within the c-myc gene is thought to play a maqjor role in determining levels of c-myc mRNA in both normal and tumor cells. A discrete site of blockage to transcriptional elongation has previously been localized at the 3' end of exon 1 of the mouse and human c-myc genes. We here identify an additional site oftranscriptional attenuation that is located between the P1 and P2 promoters of the c-myc gene and that mediates premature termination of transcripts initiating from the P1 promoter. A 95-nucleotide DNA fragment derived from this region prematurely terminated transcription when placed downstream from the promoter of the H-2Kb^l gene and assayed by expression in Xenopus oocytes. We also show that the previously identified attenuation signal in exon 1 of the mouse c-myc gene can mediate premature termination of Pl-initiated tran-