The vomeronasal organ comprises a pair of narrow tubes in the mammalian nasal septum, serving as a chemosensory system for pheromones. We examined the expression and localization of water channel aquaporins (AQPs) in the rat vomeronasal organ. AQP1 was localized in blood vessels, being particularly abundant in cavernous tissues of the nonsensory mucosa. AQP5 was found in the apical membrane of the gland acinar cells in the vomeronasal organ. AQP3 was detected in the basal cells of the nonsensory epithelium, whereas it was absent in the sensory epithelium. AQP4 was found in both the sensory and the nonsensory epithelia. Interestingly, AQP4 was highly concentrated in the sensory cells of the sensory epithelium. Immunoelectron microscopic examination clearly showed that AQP4 was localized at the plasma membrane in the cell body and lateral membrane of the dendrite, except for the microvillous apical membrane. Nerve fiber bundles emanating from neuronal sensory cells were positive for AQP4, whereby the plasma membrane of each axon was positive for AQP4. These observations clearly show that neuronal sensory cells in the vomeronasal organ are unique in that they express abundant AQP4 at their plasma membrane. This is in marked contrast to the olfactory and central nervous systems, where AQPs are not detectable in neurons, and instead, AQP4 is abundant in the supporting cells and astrocytes surrounding them. The present findings suggest a unique water-handling feature in neuronal sensory cells in the vomeronasal organ.