Na+ currents with tetrodotoxin resistance (TTX-R) have been observed in neurons, but the full-length cDNAs encoding the TTX-R Nav1.5 channels in human and rat brains have not been identified. In this study, four full-length cDNAs encoding the alpha-subunits of the Nav1.5 channels in human and rat cerebral cortexes were cloned and designated hB1, hB2, rN1 and rN2 (accession number: EF629346, EF629347, EF618549, EF618550). The longest open reading frame of hB1 or rN1 encodes 2016 amino acid residues. Sequence analysis has indicated that hB1 is highly homologus with human cardiac Nav1.5/SCN5A (hH1) with >98% amino acid identity. Genomic sequence analysis of Nav1.5/SCN5A revealed that it is exon6A rather than exon6 splice variant of Nav1.5 which is expressed in human and rat brains. Alternative splicing variants hB2 and rN2, which lack exon24 and encode proteins of 1998 amino acids, were also identified. Furthermore, the total Nav1.5 mRNA and Navbeta1 mRNA were detected in 16 different tissue types of developing Wistar rats by reverse polymerase chain reaction (RT-PCR), and their expression patterns varied among different tissue types with age development. These results suggest that Nav1.5 channels in human and rat brains are encoded by new variants of Nav1.5/SCN5A and Nav1.5 is more widely distributed and expressed than previously thought.