H ypertension is a clinical syndrome characterized by increased vascular tone. However, the molecular mechanisms underlying vascular dysfunction during acquired hypertension remain unresolved. Localized intracellular Ca²⁺ release events through ryanodine receptors (Ca²⁺ sparks) in the sarcoplasmic reticulum are tightly coupled to the activation of large-conductance, Ca²⁺-activated K⁺ (BK) channels to provide a hyperpolarizing influence that opposes vasoconstriction. In this study we tested the hypothesis that a reduction in Ca²⁺ spark–BK channel coupling underlies vascular smooth muscle dysfunction during acquired hypertension. We found that in hypertension, expression of the β1 subunit was decreased relative to the pore-forming α subunit of the BK channel. Consequently, the BK channels were functionally uncoupled from Ca²⁺ sparks. Consistent with this, the contribution of BK channels to vascular tone was reduced during hypertension. We conclude that downregulation of the β1 subunit of the BK channel contributes to vascular dysfunction in hypertension. These results support the novel concept that changes in BK channel subunit composition regulate arterial smooth muscle function.