cDNAs encoding large-conductance Ca 2+ -activated K + channel a-subunit (rSlo) were obtained from rat brain. From the DNA sequence of multiple rslo clones, we identified a specific sequence variation of 81 nucleotides, which is either absent from or present at the N-terminal region of a putative Ca 2+ -sensing domain of the channel. Transcripts containing such variations were detected in different ratios from several brain regions, and their functional significance was further examined. When heterologously expressed in Xenopus oocytes, both rSlo variants, named rSlo 0 and rSlo 27 , generated Ca 2+ -activated and voltage-activated K + currents characteristic of neuronal large-conductance Ca 2+ -activated K + (BK Ca ) channels. Single-channel recordings of the two channels showed almost identical permeation characteristics and steady-state gating behavior. Noticeable differences between rSlo 0 and rSlo 27 were revealed when the macroscopic currents were measured at various voltages and intracellular Ca 2+ concentrations. rSlo 27 activated was more rapidly than rSlo 0 in the presence of the same voltage stimulus, and the differences in these activation kinetics were dependent on the concentration of intracellular Ca 2+ . Despite their similar apparent affinities for Ca 2+ , rSlo 0 and rSlo 27 showed significant differences in their co-operative gating behavior. The Hill coefficient for intracellular Ca 2 was estimated to be about 3.7 for rSlo 27 regardless of the membrane voltage, and that for rSlo 0 was reduced from about 5 to 2 as the membrane voltage changed from 40 to 140 mV. As activation of BK Ca channels is involved in rapid hyperpolarization of action potentials, the differential processing of rslo transcripts, and the generation of channels with different activation kinetics and Ca 2+ cooperativity may be a mechanism for tuning the excitability of neurons in different brain regions.