We examined factors important in regulation of expression of the Na+/H+exchanger gene in NIH/3T3 cells. A stable fibroblast cell line was generated that contained a 1.1-kb proximal fragment of the mouse NHE1 promoter. The addition of serum to serum-starved cells resulted in an increase in activity of the NHE1 promoter. The mitogenic agonists insulin, thrombin, and epidermal growth factor also increased transcription from the NHE1 promoter. Phorbol esters also increased NHE1 promoter-directed transcription, whereas the serine/threonine protein kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine inhibited this stimulation. The protein kinase inhibitors GF-109203X, PD-98059, and genistein all stimulated promoter activity. Promoter deletion analysis and gel mobility shift assays showed that a region between 0.9 and 1.1 kb from the start site was involved in mediating the effect of mitogenic stimulation. The results show that a variety of mitogenic factors can activate the NHE1 promoter during cell growth and proliferation.