Recent work from this laboratory has demonstrated that purinergic-mediated depolarization of human microglia inhibited a store-operated pathway for entry of Ca 2+ . We have used Fura-2 spectrofluorometry to investigate the effects on store-operated Ca 2+ influx induced by replacement of NaCI with Na-gluconate in extracellular solutions. Three separate procedures were used to activate store-operated channels. Platelet activating factor (PAF) was used to generate a sustained influx of Ca 2+ in standard physiological saline solution (PSS). The magnitude of this response was depressed by 70% after replacement of PSS with low CI – PSS. A second procedure used ATP, initially applied in Ca 2+ -free PSS solution to deplete intracellular stores. The subsequent perfusion of PSS solution containing Ca 2+ resulted in a large and sustained entry of Ca 2+ , which was inhibited by 75% with low Cl – PSS. The SERCA inhibitor cyclopiazonic acid (CPA) was used to directly deplete stores in zero-Ca 2+ PSS. Following the introduction of PSS containing Ca 2+ , a maintained stores-operated influx of Ca 2+ was evident which was inhibited by 77% in the presence of the low Cl – PSS. Ca 2+ influx was linearly reduced with cell depolarization in elevated K + (7.5 to 35mM) suggesting that changes in external Cl – were manifest as altered electrical driving force for Ca 2+ entry. However, 50mM external KCl effectively eliminated divalent entry which may indicate inactivation of this pathway with high magnitudes of depolarization. Patch clamp studies showed low Cl – PSS to cause depolarizing shifts in both holding currents and reversal potentials of currents activated with voltage ramps. The results demonstrate that Cl – channels play an important role in regulating store-operated entry of Ca 2+ in human microglia.