The blood supply and osseocartilaginous support structures of the vomeronasal organ of the rat were studied. The study focused on adults, though 3- to 18-day-old animals were also examined. The techniques used included dissection and microdissection, injection of the vascular system with Araldite or with Indian ink in agar or gelatine, conventional histology, and scanning and transmission electron microscopy. The results indicated that blood reaches the vomeronasal organ via a branch of the sphenopalatine artery, and drains into an associated vein. Within the organ, one vein stood out by virtue of its size; this vein is accompanied by lesser veins, together with arterioles, capillaries, and lymphatic vessels. Connective tissue was readily apparent, though its distribution was heterogeneous. Analysis of series of transverse sections indicates that, in adults, the capsule that encases the vomeronasal organ is bony; in younger animals, the capsule is bony externally and cartilaginous internally; in very young animals, the capsule is entirely cartilaginous. However, it was noted that the change from cartilage to bone was due not to ossification of the existing cartilage, but to physical displacement of that cartilage by an extension of the vomer and incisive bones. Taken together, these results confirm the importance of considering the morphology of the vomeronasal organ as a whole, since there are major changes from rostral to caudal ends. Secondly, our findings regarding blood supply and the nature of the capsule support the view that the vomeronasal organ acts as a pump.