THE effects of the membrane permeant Ca2+ chelator BAPTA-AM on voltage-gated Na+, Ca2+, K+ (INa, ICa, IK, respectively) and Ca2+-activated K+ (IKCa) currents in cultured bovine chromaffin cells were investigated using the whole-cell patch-clamp technique. Superfusion with BAPTA-AM (50 μM) induced a rapid (< 60 s) and reversible block of both IKCa and IK (∼50%), without affecting either ICa or INa. Preincubation with BAPTAAM (50 μM, 30 min) or cell loading with the non-permeable active form of BAPTA (10 mM in the pipette solution) permanently blocked IKCa. BAPTA-AM superfusion (50 μM) also blocked IK (∼53%) after BAPTA-loading or BAPTA-AM preincubation. In conclusion, we show a fast and reversible block of IKCa and IKby BAPTA-AM, acting directly on K+ channels before it operates as a Ca2+ chelator, in cultured bovine chromaffin cells.