The olfactory area of the nasal cavity is lined with olfactory receptor cell cilia that come in contact with incoming odor molecules. Ultrastructural immunocytochemical studies in rodents have shown that these cilia contain all the proteins necessary to transduce the odorous message into an electrical signal that can be transmitted to the brain. These signaling proteins include putative odor receptors, GTP binding proteins, type III adenylyl cyclase and cyclic nucleotide-gated channels. The rest of the cells, including dendrites and dendritic knobs, showed no discernible labeling with antibodies to these signaling proteins. Furthermore, freeze-fracture and freeze-etch studies have shown that the membrane morphology of olfactory cilia differs substantially from that of non-sensory cilia. Olfactory cilia have many more membrane particles. Transmembrane signaling proteins, such as odor receptors, adenylyl cyclase and cyclic nucleotide-gated channels, conceivably appear as membrane particles. Thus, the long-standing supposition that olfactory cilia are peculiarly adapted to deal with the reception and initial transduction of odorous messages has now been verified in terms of both ultrastructural morphology and cytochemistry. Emerging studies on vomeronasal receptor cell microvilli indicate that the same is true for this organ, even though the actual signaling components differ from those of the main olfactory system.