In the present study we investigated whether transcription, alternative splicing and developmental regulation of voltage-gated Na(+) channels occur in a species-dependent manner in the mammalian heart. The composition of the Na(+) channel transcript pool including Na(v)1.1-Na(v)1.5 was analysed by RT-PCR in mouse, rat, pig, and human hearts. We found that relative transcript levels of tetrodotoxin (TTX) sensitive channels (Na(v)1.1-Na(v)1.4) decreased with increasing heart size (30% for mouse, 8% for rat, and 4% for pig/human). Considering transcript levels of individual isoforms, human and pig hearts were nearly indistinguishable whereas large differences existed between human and mouse. We also noticed that alternative splicing and age-dependent Na(+) channel expression occurred in a species-dependent manner. Unexpectedly, we even observed gender differences in the cardiac levels of TTX sensitive Na(+) channels in humans. Our data suggest that species differences could underlie published discrepancies on the role of TTX sensitive Na(+) channels in the heart and that the functions of those minor cardiac isoforms in normal and diseased hearts are best studied in larger mammalian animals.