The vomeronasal organ (VNO) is a chemoreceptor organ enclosed in a cartilaginous capsule and separated from the main olfactory epithelium. The vomeronasal neurons have two distinct types of receptor that differ from each other and from the large family of odorant receptors. The VNO receptors are seven-transmembrane receptors coupled to GTP-binding protein, but appear to activate inositol 1,4,5-trisphosphate signaling as opposed to cyclic adenosine monophosphate. The nature of stimulus access suggests that the VNO responds to nonvolatile cues, leading to activation of the hypothalamus by way of the accessory olfactory bulb and amygdala. The areas of hypothalamus innervated regulate reproductive, defensive, and ingestive behavior as well as neuroendocrine secretion.