ATP receptors have been found on several types of supporting cells in the organ of Corti (1) and the associated signalling pathways have been proposed to play a protective role in noise-induced damage in the cochlea (2). Border cells are one class of cochlear supporting cell that surround the sensory inner hair cells (IHCs) and thus the first auditory synapse. Here we investigate the effects of ATP on border cells in the adult mouse cochlea and suggest a mechanism through which purinergic receptor activation of border cells could affect the IHC sensitivity to sound.The temporal bones of adult mice (ages P30-P90) were removed from animals killed by cervical dislocation, glued to the recording chamber and the organ of Corti was exposed through an opening at the apical end of the cochlea. Whole cell tight seal patch clamp recordings were made from border cells initially identified visually. Cells were decoupled either with flufenamic acid (100 μM) or with 1-octanol (1 mM) to improve voltage clamp. 10 kDa dextran conjugated fluorescein was added to the patch pipette (0.2% w/v) to confirm recordings were from border cells. For Ca2+ imaging experiments, cochleas were incubated for 40 min in 20 μM OGB1-AM, washed and then fluorescent signals were collected by widefield epifluorescence microscopy. For immunochemistry, cochleas were fixed in 4% PFA and then labelled with antibodies against P2X2 (Alomone Labs). Stimulation of border cells with 100 μM ATP elicited an inward current with two components. An early, faster component had characteristics consistent with P2X2/3 receptor activation with amplitude 393 ± 81 pA (SEM, n = 10). It desensitized with τ1/2 = 0.9 s. The presence of P2X2 receptors on border cells was confirmed by immunostaining. A later, slow inward component (amplitude 344 ± 72 pA n=10) also desensitized and was blocked by 100 µM flufenamic acid (n = 6). 10 mM BAPTA in the patch pipette also blocked this slow component (n = 7). The second component current’s reversal potential shifted from +4.2 ± 2.8 mV (SEM, n = 7) in bilateral Cl- recordings to +27.7 ± 7.5 mV (n = 9) when the pipette contained 130 mM SCN-. Taken together these results are consistent with the second component being a Ca2+-dependent Cl- channel. Stimulation of cells with 100 μM UTP, a P2Y agonist, did not activate any conductance even though it was sufficient to elicit an increase in cytosolic Ca2+ in the supporting cells surrounding IHCs, when measured with calcium indicators. We therefore find that calcium influx through P2X receptors in border cells is sufficient to open a Ca2+-activated Cl- current. We suggest that Ca2+-dependent activation of the chloride channel could contribute to the regulation of the extracellular volume and thus affect the membrane potential of the sensory IHC.