The molecular mechanisms underlying increased arterial tone during hypertension are unclear. In vascular smooth muscle, localized Ca²⁺ release events through ryanodine-sensitive channels located in the sarcoplasmic reticulum (Ca²⁺ sparks) activate large-conductance, Ca²⁺-sensitive K⁺ (BK) channels. Ca²⁺ sparks and BK channels provide a negative feedback mechanism that hyperpolarizes smooth muscle and thereby opposes vasoconstriction. In this study, we examined Ca²⁺ sparks and BK channel function in Wistar-Kyoto (WKY) rats with borderline hypertension and in spontaneously hypertensive rats (SHR), a widely used genetic model of severe hypertension. We found that the amplitude of spontaneous BK currents in WKY and SHR cells were smaller than in normotensive cells even though Ca²⁺ sparks were of similar magnitude. BK channels in WKY and SHR cells were less sensitive to physiological changes in intracellular Ca²⁺ than normotensive cells. Our data indicate that decreased expression of the BK channel β1 subunit underlies the lower Ca²⁺ sensitivity of BK channels in SHR and WKY myocytes. We conclude that the lower expression of the β1 subunit during genetic borderline and severe hypertension reduced BK channel activity by decreasing the sensitivity of these channels to physiological changes in Ca²⁺. These results support the view that changes in the molecular composition of BK channels may be a fundamental event contributing to the development of vascular dysfunction during hypertension.