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The steady-state and kinetic properties of the KCNMB3 regulatory subunits associated with calcium-activated potassium channels (BK channels) are presented. BK channels containing four sequence variants (V1–V4) in the four different isoforms of the β-subunit (β3a, β3b, β3c, and β3d) were expressed in Xenopus oocytes. Reconstituted BK channel inactivation ranged from none to around 90% inactivation. In particular, channels expressing the β3b-V4 variant displayed a right shift in the potassium current voltage-dependence of activation and inactivated to about 30% of the maximum conductance, compared with wild-type β3b channels that showed no inactivation. When the membrane potential was depolarized, BK channels inactivated with a very rapid time course (∼2–6 ms). This same variant was previously demonstrated to show subtly higher incidence in patients with idiopathic epilepsy (IE) compared with …