The authors review the function and anatomy of the vomeronasal organ (organ of Jacobson), a chemosensory organ situated in the nose. In mammals, stimulation of the vomeronasal organ induces sexual behaviour and changes the hormonal status of males and females. Impairment of the vomeronasal organ in rats disrupts both the females' ultrasound calling during oestrus and their maternal behaviour. Stimulation of the vomeronasal organ promotes sexual maturation of juvenile females and induces abortion and production of pheromones. Earlier on, the vomeronasal organ in man was believed to be present only at the foetal stage; in 1991 its presence was reestablished in adults. Electrophysiological studies show that the vomeronasal organ of man is stimulated by steroids found in human skin. The physiological properties of the receptor cells of the vomeronasal organ are different from those of the olfactory organ.