Apical low-conductance SK and high-conductance Ca²⁺-activated BK channels are present in distal nephron, including the cortical collecting duct (CCD). Flow-stimulated net K secretion (J_K) in the CCD is 1) blocked by iberiotoxin, an inhibitor of BK but not SK channels, and 2) associated with an increase in [Ca²⁺]_i, leading us to conclude that BK channels mediate flow-stimulated J_K. To examine the Ca²⁺ dependence and sources of Ca²⁺ contributing to flow-stimulated J_K, J_K and net Na absorption (J_Na) were measured at slow (∼1) and fast (∼5 nl·min⁻¹·mm⁻¹) flow rates in rabbit CCDs microperfused in the absence of luminal Ca²⁺ or after pretreatment with BAPTA-AM to chelate intracellular Ca²⁺, 2-aminoethoxydiphenyl borate (2-APB), to inhibit the inositol 1,4,5-trisphosphate (IP₃) receptor or thapsigargin to deplete internal stores. These treatments, which do not affect flow-stimulated J_Na (Morimoto et al. Am J Physiol Renal Physiol 291: F663–F669, 2006), inhibited flow-stimulated J_K. Increases in [Ca²⁺]_i stimulate exocytosis. To test whether flow induces exocytic insertion of preformed BK channels into the apical membrane, CCDs were pretreated with 10 μM colchicine (COL) to disrupt microtubule function or 5 μg/ml brefeldin-A (BFA) to inhibit delivery of channels from the intracellular pool to the plasma membrane. Both agents inhibited flow-stimulated J_K but not J_Na (Morimoto et al. Am J Physiol Renal Physiol 291: F663–F669, 2006), although COL but not BFA also blocked the flow-induced [Ca²⁺]_i transient. We thus speculate that BK channel-mediated, flow-stimulated J_K requires an increase in [Ca²⁺]_i due, in part, to luminal Ca²⁺ entry and ER Ca²⁺ release, microtubule integrity, and exocytic insertion of preformed channels into the apical membrane.