REISSNER'S membrane forms a partition between the endolymphatic and perilymphatic cochlear compartments. Expression of the P2X2receptor subunit which assembles to form ATP-gated ion channels was detected in guinea-pig Reissner's membrane using the reverse transcription polymerase chain reaction (RT-PCR). The P2X2 receptor subunit protein was localized to the epithelial cells which line the endolymphatic surface of Reissner's membrane using confocal immunofluorescence. The P2X receptor expression in Reissner's membrane was functionally confirmed using whole-cell voltage-clamp. An inwardly rectifying conductance was activated in Reissner's membrane epithelial cells in the presence of extracellular ATP (100 μM). This conductance had a pharmacological profile compatible with the P2X2receptor designation, but exhibited substantial desensitisation which may be attributable to additional P2X receptor subunits. This study indicates that extra-cellular ATP, a humoral factor within scala media, acts via ATP-gated ion channels expressed by Reissner's membrane epithelial cells to decrease the driving force for sound transduction.