The in vivo effects of benzalkonium chloride, which is a preservative in most nasal sprays and drops, have been investigated in normal human volunteers. Saccharin clearance time was slightly prolonged 10 min after 0.02% benzalkonium chloride was applied, compared to that following 0.9% saline (n = 27, P = 0.04, Wilcoxon test). Sixty-five normal volunteers were randomly assigned to receive saline, fluticasone propionate aqueous nasal spray or placebo (which contained all the ingredients of fluticasone aqueous spray incl. 0.02% benzalkonium chloride, minus the fluticasone propionate) for 2 weeks, two puffs twice a day on a double-blind basis. Symptom scores, acoustic rhinometry, saccharin clearance time and ciliary beat frequency were measured immediately prior to this study and again at 2 weeks. Fifty-eight individuals completed the study with >80% compliance. There was no significant difference between the three groups in any of the variables tested. Benzalkonium chloride causes slight prolongation of mucociliary clearance shortly after application but has no detectable effect on nasal mucosal function after 2 weeks regular use.