The recognition of odorants by olfactory receptors represents the first stage in odor discrimination. Here, we report the generation of an expression library containing a large and diverse repertoire of mouse olfactory receptor sequences in the transmembrane II-VII region. From this library, 80 chimeric receptors were tested against 26 odorants after transfection into HEK-293 cells. Three receptors were identified to respond to micromolecular concentrations of carvone, (-) citronellal, and limonene, respectively. We also found that the mouse I7 receptor, unlike the rat I7 receptor, prefers heptanal instead of octanal, as a result of a single valine-to-isoleucine substitution. This finding represents the beginning of a molecular understanding of odorant recognition. The identification, on a large scale, of cognate receptor-odorant interactions should provide insight into olfactory coding mechanisms.