The vomeronasal system is one of the chemosensory systems. It is believed to play an important role in the processing of pheromonal signals and the expression of reproductive function. The sensory mechanism in the vomeronasal system has not been studied as well as those in the other chemosensory systems such as the main olfactory system and the gustatory system. Recently information on the neuronal development and plasticity in the vomeronasal system has gradually increased. Neuronal development in the rat vomeronasal system continues until about the 4th week after birth. The vomeronasal system exhibits a high degree of plasticity. In this article we shall review neuronal development and plasticity in the mammalian vomeronasal system.