Electron microscopic studies of the human nasal mucosa have complemented earlier histological and histochemical studies. In this paper, the fine structural details observed in different tissue components are described and discussed in relation to the function of the nasal mucosa under normal and pathological conditions. The physiological changes in the anterior part of the inferior turbinate which occur under the influence of air currents differ clearly from the electron microscopic picture of respiratory mucosa with typical ciliated epithelium. The fine structure of the allergic nasal mucosa differs according to the stage of the allergic reaction; the characteristic changes are described both after different time intervals following intranasal allergic testing with positive reaction as well as in patients with seasonal or perennial nasal allergy of longer duration.