SPIRAL ganglion neurones in rat cochlea express three different isoforms of the P2X2receptor subunit which assemble into ATP-gated ion channels. Two of these P2X2R subunit isoforms have previously been detected in other auditory tissues. The third isoform (designated P2X2–3R) has not been described. This isoform lacks 39 bp immediately prior to the stop codon, corresponding to a 13 amino acid deletion of the extreme C-terminus domain. Using directin situRT-PCR, expression of P2X2R mRNA was confined to a subpopulation of type I spiral ganglion neurones. This study supports a role for extracellular ATP as a neurotransmitter for a discrete population of auditory neurones where variation in P2X2R isoform assembly may confer functional heterogeneity, including enhanced desensitization.