We isolated two full-length cDNA clones from the adult murine heart that encode two different voltage-gated Na+ channels: mH1 and mH2. Sequence comparisons indicated that mH1 is highly homologous to rat SCN5A, whereas mH2 is highly homologous to SCN4A, expressed in rat skeletal muscle. Electrophysiological properties of mH1 channels strongly resembled the tetrodotoxin (TTX)-resistant Na+ current of mouse ventricular cells, whereas mH2 channels activated at more positive potentials and were highly sensitive to TTX [50% inhibitory constant (IC50) = 11 nM]. We found that mH2 is not expressed in cardiac cells of neonatal mice, but appears to be upregulated during the development. Besides these Na+ channel isoforms, we also detected two alternatively spliced mH1 variants that were characterized by deletions within the sequence coding for the intracellular loop between domains II and III. One of the shortened channels, mH1-2, developed Na+ currents indistinguishable from those of mH1. The other splice variant (mH1-3) did not form functional channels. Quantitative reverse transcriptase-polymerase chain reaction indicated that RNA preparations of the adult mouse heart contain 54% mH1, 25% mH1-2, 16% mH2, and 5% mH1-3. Conclusively, mH1 generates the main portion of the mouse cardiac TTX-resistant Na+ current and mH2 is a candidate for TTX-sensitive currents previously described in adult cardiomyocytes. Furthermore, the presence of mH1-2 and mH1-3 transcripts indicates that alternative splicing plays a role in the regulation of functional Na+ channels in cardiomyocytes.