Earlier work has shown that in vitro myogenesis of muscle cells is accompanied by important changes in the expression of NCAM mRNAs and proteins, consisting mainly in an upregulation of the smaller transcripts and the 120- to 125-kD NCAM isoforms they encode. By measuring run-on transcription rates in nuclei isolated from C2 myoblasts and myotubes, we show in this report that transcriptional control can account for the increase in NCAM mRNA levels during myogenesis. Hence, the quantitative changes in overall abundance of NCAM transcripts and the concomitant qualitative changes in the mRNA pattern are regulated by independent mechanisms. Moreover, as transcriptional activity was stimulated to similar extents in the 5' and 3' region of the gene, we can exclude premature transcription termination as a mechanism involved in the preferential synthesis of shorter transcripts. Further support for transcriptional regulation of the NCAM gene is provided by experiments showing increased NCAM promoter activity in differentiating cells. As this increase was observed with 4.5 kb but not with 426 bp of upstream sequence, it seems to involve one or more upstream regulatory elements.