Although K+ channels activated by Ca2+ have long been known to shape neuronal excitability, evidence is accumulating that K+ channels sensitive to intracellular Na+, termed K(Na) channels, have an equally significant role. K(Na) channels contribute to adaptation of firing rate and to slow afterhyperpolarizations that follow repetitive firing. In certain neurons, they also appear to be activated by Na+ influx accompanying a single spike. Two genes encoding these channels, Slick and Slack, are expressed throughout the brain. The spatial localization of K(Na) channels along axons, dendrites and somata appears to be highly cell-type specific. Their molecular properties also suggest that these channels contribute to the response of neurons to hypoxia.