Arterial smooth muscle cell large-conductance calcium (Ca²⁺)-activated potassium (K_Ca) channels have been implicated in modulating hypoxic dilation of systemic arteries, although this is controversial. K_Ca channel activity in arterial smooth muscle cells is controlled by localized intracellular Ca²⁺ transients, termed Ca²⁺ sparks, but hypoxic regulation of Ca²⁺ sparks and K_Ca channel activation by Ca²⁺ sparks has not been investigated. We report here that in voltage-clamped (-40 mV) cerebral artery smooth muscle cells, a reduction in dissolved O₂ partial pressure from 150 to 15 mmHg reversibly decreased Ca²⁺ spark-induced transient K_Ca current frequency and amplitude to 61 and 76 % of control, respectively. In contrast, hypoxia did not alter Ca²⁺ spark frequency, amplitude, global intracellular Ca²⁺ concentration, or SR Ca²⁺ load. Hypoxia reduced transient K_Ca current frequency by decreasing the percentage of Ca²⁺ sparks that activated a transient K_Ca current from 89 to 63 %. Hypoxia reduced transient K_Ca current amplitude by attenuating the amplitude relationship between Ca²⁺ sparks that remained coupled and the evoked transient K_Ca currents. Consistent with these data, in inside-out patches at -40 mV, hypoxia reduced K_Ca channel apparent Ca²⁺ sensitivity and increased the Kd for Ca²⁺ from ~17 to 32 µM, but did not alter single channel amplitude. In summary, data indicate that hypoxia reduces K_Ca channel apparent Ca²⁺ sensitivity via a mechanism that is independent of cytosolic signaling messengers, and this leads to uncoupling of K_Ca channels from Ca²⁺ sparks. Transient K_Ca current inhibition due to uncoupling would oppose hypoxic cerebrovascular dilation.