In contrast to land vertebrates, many types of biologically relevant chemical sensory stimuli have been both identified and structurally characterized in fish (especially the Cyprinidae) along with their effective concentrations. Another great advantage of fish as experimental models for the study of olfaction is their slightly less differentiated anatomy, including a number of unique experimental prerequisites: the olfactory epithelium is readily accessible and contains both microvillous and ciliated receptor neurons (Hansen et al., 1999); in contrast to terrestrial vertebrates, different types of receptor neurons are more equally distributed over single lamellae and the mid-line raphe; odours can be applied in a constant flow to the epithelium while the experimental animal is artificially respirated through the mouth and gills.