Discrimination among the vast array of odors requires that the brain discern which of the numerous odorant receptors have been activated. If individual olfactory neurons express only a subset of the odorant receptor repertoire, then the nature of a given odorant can be discerned by identifying which cells have been activated. We performed in situ hybridization experiments demonstrating that individual olfactory neurons express different complements of odorant receptors and are therefore functionally distinct. Thus, a topographic map, defining either the positions of specific neurons in the epithelium or the positions of their projections, may be employed to determine the quality of an olfactory stimulus. Neurons expressing specific receptors appear to be randomly distributed within the olfactory epithelium. These data are consistent with a model in which randomly dispersed olfactory neurons with common receptor specificities project to common glomeruli in the olfactory bulb.