Voltage-gated sodium channels consist of pore-forming α-subunits and auxilliary β-subunits. They are responsible for the rapid rising phase of the cardiac action potential. The subunit composition and distribution of cardiac sodium channels remains somewhat elusive. Ventricular cardiomyocytes were isolated from humanely killed neonatal rats (1 – 2 days old) and stained with antibodies specific for sodium channel α- and β-subunits (n > 15 each; for details see Maier et al. 2002). Connexin 43 (Cx43) and α-aktinin were used as markers for intercalated discs (i.e. cell-cell contacts) and intracellular sarcomeres, respectively. Whole cell voltage clamp experiments were performed for determination of TTX dose-response effects (for details see Gaughan et al. 1998). Our data demonstrate that the neuronal tetrodotoxin (TTX)-sensitive sodium channel α-subunit isoforms Nav1.1, Nav1.2, Nav1.3 and Nav1.6 are expressed in neonatal rat heart additionally to the predominant cardiac α-subunit isoform, Nav1.5. To the best of our knowledge, this is the first demonstration of Nav1.2 expression in the heart. Also, we provide the first evidence for the existence of all four β-subunits (β1–β4) in neonatal cardiac cells. All present subunits are uniformly distributed across the membrane surface of neonatal cardiomyocytes and cluster at the intercalated discs as demonstrated by double immunoflourescence labelling. For complementation of the above mentioned localisation studies we performed electrophysiological studies using the whole cell configuration of the patch clamp technique in order to provide the first functional evidence of the presence of neuronal-type, TTX sensitive sodium current in ventricular myocardium under physiological conditions. Contrary to the distribution in adult cardiomyocytes, neuronal TTX-sensitive sodium channel isoforms are localized uniformly across the sarcolemma and do not show isoform-specific compartmentation. TTX-sensitive neuronal sodium channels are functionally expressed in neonatal myocytes and clustering of sodium channel subunits at the intercalated discs suggests a specific function in propagation of the cardiac action potential from cell to cell to secure proper excitation propagation throughout the myocardium.