Nasal obstruction, a prominent feature of rhinitis, may be quantified in humans by haemodynamic techniques (measuring local blood flux), static methods (measuring the geometry of nasal cavities) and dynamic methods (assessing the patency of nasal airways through the measure of resistance to air flow). These methods demonstrated the nasal decongestant activity of xylometazoline in healthy volunteers and rhinitis patients. Controlled double-blind studies established the clinical efficacy of xylometazoline in infectious and allergic (seasonal and perennial) rhinitis versus placebo and in comparison with various reference substances. The effects on nasal epithelium ciliary activity which are observed in vitro are modest and even less pronounced in vivo owing to dilution in situ and protective physiological processes.