We have cloned and expressed nine Ca(2+)-activated K+ channel isoforms from human brain. The open reading frames encode proteins ranging from 1154 to 1195 amino acids, and all possess significant identity with the slowpoke gene products in Drosophila and mouse. All isoforms are generated by alternative RNA splicing of a single gene on chromosome 10 at band q22.3 (hslo). RNA splicing occurs at four sites located in the carboxy-terminal portion of the protein and gives rise to at least nine ion channel constructs (hbr1-hbr9). hslo mRNA is expressed abundantly in human brain, and individual isoforms show unique expression patterns. Expression of hslo mRNA in Xenopus oocytes produces robust voltage and Ca(2+)-activated K+ currents. Splice variants differ significantly in their Ca2+ sensitivity, suggesting a broad functional role for these channels in the regulation of neuronal excitability.