Skeletal muscle is a contractile tissue which expresses a wide variety of ion channels. As many other physiological processes, skeletal muscle formation takes place as a result of several complex changes at both cellular and molecular level. During this process the membrane potential changes from -10mV to -70mV. This hyperpolarization is generated by the coupled action of different types of ion channels. It has been described that some of those proteins are directly involved in muscle fibre growth modulation. Voltage-dependent sodium channels(Nav) are one of the most important channels in the initial steps of muscle action potential propagation. Therefore, we decided to analyse the physiological regulation of Nav channels during proliferation and myogenesis in neonatal myocyte cell line L6E9. We found that myoblasts express the isoforms Nav1.4 and Nav1.5. Electrophysiological and pharmacological properties, as well as mRNA expression indicated that both isoforms were induced during myoblast proliferation. By using Tetradotoxin(TTX) we demonstrated that either Nav1.4 or Nav1.5 had no apparent role. On the other hand, myotube differentiation selectively induced Nav1.5, accounting for almost 90% of Na⁺ current in myotubes. Unlike proliferation, this protein would play a pivotal role in myogenesis. Differentiation also increased colocalization of Nav1.5 in caveolar rafts. Finally, we demonstrated that the treatment of myoblasts with Neuregulins(NRG), wich induce myotube formation, further increased Nav1.5 expression. Our results indicate a differential physiological role of the two isoforms of Nav expressed in L6E9 cell line during myoblasts proliferation and myotube differentiation. Supported by BFU 2005-00695(Ministerio de Educación y Ciencia,Spain).