Cynomolgus monkeys were treated for 28 days with fluticasone propionate aqueous nasal spray (FPANS) 8 × 0.1 ml/day, which contained 0.02% benzalkonium chloride (BKC). Rats were treated for 1 hour/day for 28 days with a beclomethasone dipropionate aqueous nasal spray (BDPANS) essentially similar to the commercial preparation (Allen and Hanburys Ltd, Middlesex, England), which contained 0.01% BKC. Control animals received 5% glucose (monkeys) or air (rats). Sections of nasal mucosa were examined at 4 different levels by light microscopy. Counts of ciliated cells were made and scanning and transmission electron microscopy was performed on target sites that had been directly exposed to treatment (inferior turbinate of monkeys and intermediate turbinate of rats). No reduction in the number of ciliated cells or microscopical changes were seen in the nasal epithelium of the animals, and in particular there was no damage to the ultrastructure of the ciliated cells, the cilia themselves or their basal bodies that was related to treatment. The findings were compared with the results of previousin vitroandin vivostudies. Although ciliostasis has been reported with BKC inin vitromodels, where mucus is relatively depleted, such results should be interpreted with caution since BKC repeatedly applied to the nasal respiratory epitheliumin vivohas not caused damage to the ciliated epithelial cells. The animals received greater exposure to BKC-containing corticosteroids than patients who are treated with the aqueous nasal sprays for allergic rhinitis. The lack of any morphological damage supports previous findings that no damage to ciliated cells occurs in patients treated with FPANS or BDPANS.