In this study, the activation mechanisms of the background chloride current and the role of the current in maintaining of basal cell volume were investigated in human nasopharyngeal carcinoma CNE-2Z cells.Under isotonic conditions, a background chloride current was recorded by the patch clamp technique.The current presented the properties similar to those of the volume-activated chloride current in the same cell line and was inhibited by chloride channel blockers or by cell shrinkage induced by hypertonic challenges.Extracellular applications of reactive blue 2, a purinergic receptor antagonist, suppressed the background chloride current in a concentration-dependent manner under isotonic conditions.Depletion of extracellular ATP with apyrase or inhibition of ATP release from cells by gadolinium chloride decreased the background current.Extracellular applications of micromolar concentrations of ATP activated a chloride current which was inhibited by chloride channel blockers and hypertonic solutions.Extracellular ATP could also reverse the action of gadolinium chloride.Transfection of CNE-2Z cells with ClC-3 siRNA knocked down expression of ClC-3 proteins, attenuated the background chloride current and prevented activation of the ATP-induced current.Furthermore, knock-down of ClC-3 expression or exposures of cells to ATP (10mM), the chloride channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and tamoxifen, or reactive blue 2 increased cell volume under isotonic conditions.The results suggest that C1C-3 protein may be a main component of background chloride channels which can be activated under isotonic conditions by autocrine/paracrine ATP through purinergic receptor pathways; the background current is involved in maintenance of basal cell volume.