The vomeronasal complex of the rat was studied by means of different staining techniques and light microscopy between the 13th day of gestation and 42 days of postnatal life. The anlage of the vomeronasal organ in the 13-day-old embryo consisted of a cluster of cells proliferating from the olfactory placode towards the medial line. The vomeronasal organ was well developed by the end of gestation, showing the same ratio between receptor and receptor-free epithelium as in the adult animal. However, full development of the epithelia did not occur until the end of the second postnatal week. Glandular rudiments were observed within the vomeronasal area in 17-day-old fetuses, but the gland showed no sign of activity until the 7th day of postnatal life, when a slight PAS-positive reaction was observed in their cells' cytoplasm. This PAS-positive reaction intensified considerably during the second week and corresponded to the adult picture by the end of this time. The content of the vomeronasal glands contrasted with that of the glands flowing into the nasal fossa, which did not show any PAS-positive reaction but which stained positive for alcian blue. We also studied the evolution of the blood vessels such as the capsule of the complex. This capsule showed complete ossification by the beginning of the 3rd postnatal week, with the exception of certain regions which remained cartilaginous in the adult. Some capillaries were observed in the complex in 17-day-old fetuses, and by 20 days of gestation some of them could be observed penetrating the receptor epithelium. Subsequently, the vascularization of the complex became so profuse that one could consider the vomeronasal complex to consist of erectile tissue.