Odours are perceived by vertebrates through a complex biochemical mechanism, whose elements have not been fully identified. Before reaching the cilia of the olfactory neurons, where chemical recognition is believed to occur, odorant molecules have to diffuse through a thick mucus layer, where they interact with its chemical components. These processes, occurring before the interaction with olfactory receptors, have been called "perireceptor events" (Getchell et al., 1984); they are now believed to play an important role in odour perception. Odorant-binding proteins (OBPs) represent the best characterized species involved at this stage; they are very concentrated in the mucus and reversibly bind several types of odorants; their physiological function in odour perception, however, has not been clearly defined.