The co-expression of NMDAR-1 and GluR1-4 mRNAs in spiral ganglions of rat and guinea-pig cochleas was checked using a non-radioactive in situ hybridization technique. NMDAR-1, GluR2 and GluR3 mRNAs were expressed in the large neurones (type I) of the ganglion which innervate inner hair cells (IHCs), a sensory cell type likely to use an excitatory amino acid as a neurotransmitter. The labelling was very intense with the GluR2 and GluR3 oligoprobes while it remained moderate with the NMDAR-1 oligoprobe. This is consistent with the predominant implication of AMPA/kainate receptors in the physiological and early pathophysiological aspects of the IHC neurotransmission. Spiral ganglion neurones did not express GluR1 and GluR4 mRNAs, but a glial expression of GluR4 mRNAs was observed.