The SCN5A gene encodes a voltage-sensitive sodium channel expressed in cardiac and skeletal muscle. Coding region mutations cause cardiac sudden death syndromes and conduction system failure. Polymorphisms in the 5′-sequence adjacent to the SCN5A gene have been linked to cardiac arrhythmias. We identified three alternative 5′-splice variants (1A, 1B, and 1C) of the untranslated exon 1 and two 3′-variants in the murine Scn5a mRNA. Two of the exon 1 isoforms (1B and 1C) were novel when compared with the published human and rat SCN5A sequences. Quantitative real time PCR results showed that the abundance of the isoforms varied during cardiac development. The 1A, 1B, and 1C mRNA splice variants increased 7.8 ± 1.7-fold (E1A), 6.0 ± 1.0-fold (E1B), and 20.6 ± 3.7-fold (E1C) from fetal to adult heart, respectively. Promoter deletion and luciferase reporter gene analysis using cardiac and skeletal muscle cell lines demonstrated a pattern of distinct cardiac-specific enhancer elements associated with exons 1A and 1C. In the case of exon 1C, the enhancer element appeared to be within the exon. A 5′-repressor preceded each cardiac enhancer element. We concluded that the murine Na+ channel has both 5′- and 3′-untranslated region mRNA variants that are developmentally regulated and that the promoter region contains two distinct cardiac-specific enhancer regions. The presence of homologous human splicing suggests that that these regions may be fruitful new areas of study in understanding cardiac sodium channel regulation and the genetic susceptibility to sudden death.