Following the all-or-nothing action potential (AP), a graded spike afterdepolarization (ADP) is seen in many neuron types. In CA1 pyramidal cells (PCs), a persistent Na+ current (INaP) contributes to the generation of the ADP. It has been hypothesized that a substantial portion of INaP is mediated by Nav1.6 Na+ channel subunits. We therefore used mice deficient in functional Nav1.6 subunits (Med & MedJ mice) to assess the contribution of this subunit to INaP in isolated CA1 PCs. INaP amplitude was decreased by ~75% in both Med and MedJ mice compared to littermate controls. Moreover, the resurgent current activated by repolarization was decreased in the mutant mice. In further experiments, we examined the discharge behaviour of CA1 PCs in Med mice and WT littermates in the slice preparation. The AP threshold was significantly more depolarized by ~5 mV in the mutant mice, suggesting an important role for Nav1.6 in setting AP threshold. Prolonged current injections revealed an increased spike accommodation of the first 3-4 spikes in Med mice. Surprisingly, the spike ADP was not decreased in Med mice, even though INaP participates in generating this potential in normal CA1 PCs. Our data indicate that Nav1.6 sets the AP threshold and modulates repetitive firing behaviour of CA1 PCs. We also suggest that deletion of functional Nav1.6 subunits causes adaptive changes that maintain the size of the ADP despite the marked reduction in INaP.