Ca²⁺ sparks are small, localized cytosolic Ca²⁺ transients due to Ca²⁺ release from sarcoplasmic reticulum through ryanodine receptors. In smooth muscle, Ca²⁺ sparks activate large conductance Ca²⁺-activated K⁺ channels (BK channels) in the spark microdomain, thus generating spontaneous transient outward currents (STOCs). The purpose of the present study is to determine experimentally the level of Ca²⁺ to which the BK channels are exposed during a spark. Using tight seal, whole-cell recording, we have analyzed the voltage-dependence of the STOC conductance (g_(STOC)), and compared it to the voltage-dependence of BK channel activation in excised patches in the presence of different [Ca²⁺]s. The Ca²⁺ sparks did not change in amplitude over the range of potentials of interest. In contrast, the magnitude of g_(STOC) remained roughly constant from 20 to −40 mV and then declined steeply at more negative potentials. From this and the voltage dependence of BK channel activation, we conclude that the BK channels underlying STOCs are exposed to a mean [Ca²⁺] on the order of 10 μM during a Ca²⁺ spark. The membrane area over which a concentration ≥10 μM is reached has an estimated radius of 150–300 nm, corresponding to an area which is a fraction of one square micron. Moreover, given the constraints imposed by the estimated channel density and the Ca²⁺ current during a spark, the BK channels do not appear to be uniformly distributed over the membrane but instead are found at higher density at the spark site.