Late Na⁺ current (I_NaL) is a major component of the action potential plateau in human and canine myocardium. Since I_NaL is increased in heart failure and ischemia, it represents a novel potential target for cardioprotection. However, the molecular identity of I_NaL remains unclear. Objective: We tested the hypothesis that the cardiac sodium channel (NaCh) isoform (Na_v1.5) is a major contributor to I_NaL in adult dog ventricular cardiomyocytes (VCs). Methods: Cultured VCs were exposed to an antisense morpholino-based oligonucleotide (Na_v1.5asOLIGO) targeting the region around the start codon of Na_v1.5 mRNA or a control nonsense OLIGO (nsOLIGO). Both densities of transient Na⁺ current (I_NaT) and late I_NaL (both in pA/pF) were monitored by whole cell patch clamp. Results: In HEK293 cells expressing Na_v1.5 or Na_v1.2, Na_v1.5 asOLIGO silenced functional expression of Na_v1.5 (up to 60 % of the initial I_NaT) but not Nav1.2. In both nsOLIGO-treated controls and untreated VCs, I_NaT and I_NaL remained unchanged for up to 5 days. However, both I_NaT and I_NaL exponentially decreased with a similar time course (=46 and 56h, respectively) after VCs were treated with Na_v1.5 asOLIGO without changes in 1) decay kinetics, 2) steady-state activation and inactivation, and 3) the ratio of I_NaL to I_NaT. Four days after exposure to Na_v1.5 asOLIGO, I_NaT and I_NaL amounted to 68±6% (n=20, mean±SE, P<0.01), and 60±7% (P<0.018, n=11) of those in VCs treated by nsOLIGO, respectively. Conclusion: In adult dog heart Na_v1.5 sodium channels have a "functional half-life" of ~35h (0.69t) and make a major contribution to I_NaL.