Microarray technology has provided an unprecedented opportunity to study gene expression profiles at a whole-genome level. As a first step toward a comprehensive understanding of inner ear gene expression, mouse cochleas were examined at two developmental stages (P2 and P32) using GeneChip oligonucleotide arrays. A large number of genes and ESTs (> 10,000) were found to be expressed in the cochlea. Expression profiles derived from duplicate samples at the same developmental stages showed general agreement and indicated the reproducibility of the assay. The expression of many known hair-cell genes was detected in the whole-cochlea samples, demonstrating the relatively high sensitivity of the assay. Genes highly expressed only at P2 or P32 were also identified and their expression patterns correlate with their functions in the cochlea. A web-based database with external links was set up for public access, which should facilitate the discovery of genes important in the development and function of the inner ear and should aid the identification of additional deafness genes.