The signal elicited by the interaction of odorous ligands with receptors on olfactory sensory neurons must be decoded by the brain to determine which of the numerous receptors have been activated. We have examined the patterns of odorant receptor expression in the rat olfactory epithelium to determine whether the mammalian olfactory system employs spatial segregation of sensory input to encode the identity of an odorant stimulus. In situ hybridization experiments with probes for 11 different odorant receptors demonstrate that sensory neurons expressing distinct receptors are topologically segregated into a small number of broad, yet circumscribed, zones within the olfactory epithelium. Within a given zone, however, olfactory neurons expressing a specific receptor appear to be randomly distributed, rather than spatially localized. The complex mammalian olfactory system may therefore compartmentalize the epithelium into anatomically and functionally discrete units, such that each zone expresses only a subset of the entire receptor repertoire.