A full-length K+ channel cDNA of Kv4.3, with an open reading frame of 611 amino acids, was isolated from rat hippocampus. Functional expression of Kv4.3 cDNA in Xenopus oocytes revealed an A-type K+ channel. In the central nervous system, Kv4.3 is most prominently expressed in the retrosplenial cortex, medial habenula, anterior thalamus, hippocampus, cerebellum, as well as lateral geniculate and superior colliculus, which are important for vision. The abundant expression of Kv4.3 in many CNS neurons supports its important role as a major component of subthreshold A currents in the control of action potentials and thus neuronal excitability.