Low stringency hybridization screening of a rat genomic DNA library with a previously described cDNA clone encoding a rat voltage-gated potassium channel has resulted in the characterization of a member of the potassium channel family, RGK5. An uninterrupted nucleotide sequence encodes a protein 525 amino acids in length, revealing that the entire coding region resides on a single exon. RGK5 transcripts are present in both mouse thymus and rat brain, as determined by Northern blot analysis. RNA transcribed in vitro from RGK5 genomic DNA directs the expression of functional potassium currents after injection into Xenopus oocytes. The currents are activated by depolarization, being half-activated at -14 mV, and inactivate almost completely during depolarizations of 1 to 2 s. The properties of the currents strongly resemble those of the type n potassium channel present on both immature thymocytes and Th lymphocytes.