The ability of neurons to fire precise patterns of action potentials is critical for encoding inputs and efficiently driving target neurons. At the axon initial segment and nodes of Ranvier, where nerve impulses are generated and propagated, a high density of Nav1.2 sodium channels is developmentally replaced by Nav1.6 channels. In retinal ganglion cells (GCs), this isoform switch coincides with the developmental transition from single spikes to repetitive firing. Also, Nav1.6 channels are required for repetitive spiking in cerebellar Purkinje neurons. These previous observations suggest that the developmental appearance of Nav1.6 underlies the transition to repetitive spiking in GCs. To test this possibility, we recorded from GCs ofmed(Nav1.6-null) and wild-type mice during postnatal development. By postnatal day 18, when the switch to Nav1.6 at GC initial segments is normally complete, the maximal sustained and instantaneous firing rates were lower inmedthan in wild-type GCs, demonstrating that Nav1.6 channels are necessary to attain physiologically relevant firing frequencies in GCs. However, the firing impairment was milder than that reported previously inmedPurkinje neurons, which prompted us to look for differences in compensatory sodium channel expression. Both Nav1.2 and Nav1.1 channels accumulated at initial segments and nodes ofmedGCs, sites normally occupied by Nav1.6. InmedPurkinje cells, only Nav1.1 channels were found at initial segments, whereas in other brain regions, only Nav1.2 was detected atmedinitial segments and nodes. Thus, compensatory mechanisms in channel isoform distribution are cell specific, which likely results in different firing properties.