A putative BK channel gene was cloned from a human brain (substantia nigra) cDNA library by hybridization screening. The sequence of the full length clone shows high homology with the mSlo gene, suggesting that this cDNA is the human homologue (hSlo). The hSlo clone does not contain either alternative exon A or B at its splice sites; and similar to mSlo, it has a long string of serines at its 5' end. Reverse transcription coupled with the PCR technique demonstrated the differences in expression of the isoforms among the CNS and the periphery. Expression of hSlo in Xenopus oocytes showed a family of outward currents, induced by step depolarizations, that were blocked by iberiotoxin and activated by the compound NS004, a known opener of native and cloned maxi-K channels. Single channel recordings of hSlo channels showed a high degree of voltage- and Ca(2+)-dependence, and an average single channel conductance of 285.9 pS.