Calcium/voltage-activated potassium channels (BK) control smooth muscle (SM) tone and cerebral artery diameter. They include channel-forming α and regulatory β1subunits, the latter being highly expressed in SM. Both subunits participate in steroid-induced modification of BK activity: β1provides recognition for estradiol and cholanes, resulting in BK potentiation, whereas α suffices for BK inhibition by cholesterol or pregnenolone. Aldosterone can modify cerebral artery function independently of its effects outside the brain, yet BK involvement in aldosterone’s cerebrovascular action and identification of channel subunits, possibly involved in steroid action, remains uninvestigated. Using microscale thermophoresis, we demonstrated that each subunit type presents two recognition sites for aldosterone: at 0.3 and ≥10 µM for α and at 0.3–1 µM and ≥100 µM for β1. Next, we probed aldosterone on SM BK activity and diameter of middle cerebral artery (MCA) isolated from β1−/−vs.wtmice. Data showed that β1leftward-shifted aldosterone-induced BK activation, rendering EC50~3 μM and ECMAX≥ 10 μM, at which BK activity increased by 20%. At similar concentrations, aldosterone mildly yet significantly dilated MCA independently of circulating and endothelial factors. Lastly, aldosterone-induced MCA dilation was lost in β1−/−mice. Therefore, β1enables BK activation and MCA dilation by low µM aldosterone.