The aim of this study is to analyze Scn1b mRNA expression levels and protein distribution of Scn1b, a putative modulator of the pore-forming Na(+) channel subunit in the heart, during mouse cardiac development.Scn1b mRNA levels were determined by real-time RT-PCR using embryonic hearts ranging from E9.5 to E18.5 as well as in postnatal and adult heart. Scn1b protein distribution and subcellular localization during cardiogenesis were analyzed by immunohistochemistry and confocal microscopy.Scn1b mRNA showed a dynamic expression pattern, peaking at stage E12.5 and decreasing at E15.5. Scn1b mRNA increased at later embryonic and neonatal stages, being maximal in the adult heart. Immunohistochemistry experiments revealed comparable distribution of Scn1b protein between the different cardiac chambers at early embryonic stages. With further development, Scn1b protein showed an enhanced expression in the trabeculated myocardium and the bundle branches. At the subcellular level in later embryonic and postnatal mouse cardiomyocytes, Scn1b was present in T-tubules as identified by immunostaining of alpha-actinin, and in the intercalated disks as identified by immunostaining of connexin 43.These results demonstrate that Scn1b is expressed during mouse heart development, suggesting it can play an important role in the action potential configuration of the cardiomyocytes during heart morphogenesis.