The effect on mucociliary (m.c.) activity in the rabbit maxillary sinus of cigarette smoke and ammonia (NH,) vapour delivered through a tracheal cannula or as nasal challenges was investigated by a photo-electric technique, and compared with simultaneously induced changes in the respiratory rate. Neither irritant had any effect on m.c. activity in the sinus after tracheo-bronchial exposure. However, the respiration rate was increased by NH, vapour (2.5 ml, diluted 1 : 1 with room air) and neat cigarette smoke (10 ml) by 76.9% and 24.3 % respectively (median values). In contrast, nasal challenges with both irritants increased m.c. activity by 24.1 % (NH,) and 19.1 % (cigarette smoke), and reduced the respiration rate by 31.0% (NH,) and 28.4% (cigarette smoke) (median values). NH, vapour sometimes produced an apnea proper. Identical results were obtained in laryngectomized rabbits, indicating that laryngeal afferents were not involved in the responses. Moreover, topical application of the C-fibre stimulant capsaicin mimicked the effects produced by the airway irritants. It is concluded that nasal exposure to irritants triggers at least two different protective reflexes. One is the increase of m.c. activity in the upper airways involving sensory C-fibres and the other the apneic reflex of Kratschmer.