Analysis of the Kv3 subfamily of K(+) channel subunits has lead to the discovery of a new class of neuronal voltage-gated K(+) channels characterized by positively shifted voltage dependencies and very fast deactivation rates. These properties are adaptations that allow these channels to produce currents that can specifically enable fast repolarization of action potentials without compromising spike initiation or height. The short spike duration and the rapid deactivation of the Kv3 currents after spike repolarization maximize the quick recovery of resting conditions after an action potential. Several neurons in the mammalian CNS have incorporated into their repertoire of voltage-dependent conductances a relatively large number of Kv3 channels to enable repetitive firing at high frequencies - an ability that crucially depends on the special properties of Kv3 channels and their impact on excitability.