Mammals have retained two functionally and anatomically independent collections of olfactory neurons located in the main olfactory epithelium and in the vomeronasal organ (VNO). Pheromones activate the VNO in order to elicit fixed action behaviors and neuroendocrine changes involved in animal reproduction and aggression. Differential screening of cDNA libraries constructed from individual rat VNO neurons has led to the isolation of a novel family of approximately 100 genes encoding seven transmembrane receptors with sequence similarity with Ca2+-sensing and metabotropic glutamate receptors. These genes are likely to encode a novel family of pheromone receptors. Patterns of receptor gene expression suggest that the VNO is organized into discrete and sexually dimorphic functional units that may permit segregation of pheromone signals leading to specific arrays of behaviors and neuroendocrine responses.