The complete 5' flanking region of the murine c-Ha-ras gene was cloned and sequenced. An untranslated exon (-1) was identified and the promoter region of the gene located. Like the rat and human homologues, the murine promoter is GC rich and contains several GC boxes together with a CAAT element, but lacks a TATA box, an arrangement similar to that found in many housekeeping genes. From primer extension studies, the gene was shown to have three transcriptional start sites, whose positions differ from those previously found for the human gene. No alterations in these start sites were detected between the normal gene and activated Ha-ras genes from mouse skin tumors. A region of strong homology between mouse, rat, and human Ha-ras genes exists within the large intron separating exon (-1) from the first coding exon. In addition, from chloramphenicol acetyltransferase assays, the upstream region has promoter activity which appears to be enhanced by the inclusion of sequences within this intron.