High conductance, calcium- and voltage-activated potassium (BK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (β)-subunits. β1- and β2-subunits increase the calcium sensitivity and β1-subunit also decreases the voltage dependence of the channel. We further characterized the effects of the β1- and β2-subunits on the calcium- and voltage sensitivity of the channel, analyzing the data in the context of an allosteric model for BK channel activation by calcium and voltage (Horrigan & Aldrich, 2002). For a better characterization of the effects on channel activation, a β2-subunit without its N-type inactivation domain (β2IR) was used. α- (pore-forming) and β-subunits were coexpressed in X. laevis oocytes and currents were studied with the patch clamp technique. In the absence of calcium the limiting voltage dependence for steady-state open probability, unrelated to voltage sensor movements, is not affected by any of the studied β-subunits (α alone: 0.3 electronic charges (e) , α + β1: 0.33e, α + β2IR: 0.29e). On the other hand, the maximum slope of the dln(P_O)/dV relationship, strictly related to voltage sensor-dependent activation, is significantly reduced only by the β1-subunit (α: 1.94±0.08e, α + β1: 1.40±0.3e, α + β2IR: 1.84±0.07e). When the data is fitted to the allosteric model for voltage activation, the β1-subunit reduces the voltage dependence of the voltage sensor activation (z_J) (α: 0.6e, α + β1: 0.39e, α + β2IR: 0.55e). When the allosteric model is expanded to include the activation by calcium, the reduction of z_J accounts for most of the effects of the β1-subunit on the channel steady-state activation, surprisingly including the increase of the apparent calcium sensitivity. In the case of the β2IR-subunit, an increase of the allosteric coupling factors for channel activation by calcium and voltage is required to account for its effects. The study of chimeras between β1- and β2IR-subunits shows that the differences between these β-subunits are associated to their transmembrane and/or intracellular domains. We conclude that the transmembrane and intracellular domains of the β1-subunit modulate the movement of the voltage sensors of BK channel and that the β2IR-subunit increases the channel’s apparent calcium sensitivity by a different mechanism than the β1-subunit.