The alterations in the mucociliary unit in the course of chronic inflammation of the upper respiratory tract correspond to morphologic anomalies of the respiratory epithelium and induce cuboidal and squamous metaplasia. While the squamous pattern is most probably irreversible, it is still not clear whether it is possible to restore ciliogenesis in cuboidal metaplasia. In the present study, the action of different inductors of differentiation was evaluated in vitro in isolated cells and explants from human nasal metaplastic epithelium. Polar/apolar compounds induced secretory activity, whereas retinoic acid was able to induce ciliogenesis in some cases. Therefore, the cuboidal metaplastic condition appears to be reversible, and two distinct pathways of differentiation, secretory and ciliogenetic, are identifiable.