Large-conductance Ca(2+)- and voltage-gated Slo1 BK channels are allosterically activated by depolarization and intracellular ligands such as Ca(2+). Of the two high-affinity Ca(2+) sensors present in the channel, the RCK1 sensor also mediates H(+)-dependent activation of the channel. In this study, we examined the comparative mechanisms of the channel activation by Ca(2+) and H(+). Steady-state macroscopic conductance-voltage measurements as well as single-channel openings at negative voltages where voltage-sensor activation is negligible showed that at respective saturating concentrations Ca(2+) is more effective in relative stabilization of the open conformation than H(+). Calculations using the Debye-Hückel formalism suggest that small structural changes in the RCK1 sensor, on the order of few angstroms, may accompany the H(+)-mediated opening of the channel. While the efficacy of H(+) in activation of the channel is less than that of Ca(2+), H(+) more effectively accelerates the activation kinetics when examined at the concentrations equipotent on macroscopic voltage-dependent activation. The RCK1 sensor therefore is capable of transducing the nature of the bound ligand and transmits qualitatively different information to the channel's permeation gate.