Abstract —Na + current ( I Na ) is smaller, activates and inactivates more slowly, and displays less negative voltage dependence of inactivation in the neonatal rat than in the adult rat. We have observed very similar changes when I Na is recorded as a function of time in culture in mouse atrial tumor (AT-1) cells. The differences between mature and immature I Na are reminiscent of those observed when skeletal muscle Na + channel α subunits are expressed alone (immature) or with the β 1 subunit (mature). In the present experiments, we tested the hypothesis that suppression of β 1 -subunit expression by antisense oligonucleotides would prevent the development of a mature I Na . The mouse β 1 subunit was cloned from an AT-1 cDNA library and found to be identical to that in the rat at 216/218 amino acids. AT-1 cells exposed to anti-β 1 antisense oligonucleotides displayed an immature I Na at day 8 in culture, whereas untreated cells or cells exposed to sense oligonucleotides displayed a mature I Na . This result was observed with 2 different oligonucleotides, and neither affected the rapidly activating component of the delayed rectifier K + current, another current recorded in AT-1 cells. These findings indicate that in these cells, the gating of I Na is modulated by β 1 expression and that α-β 1 coexpression is required for the development of a mature cardiac I Na phenotype.