Airway mucus is essential for the effectiveness of cough and mucociliary transport. Even the resting airway produces some mucus but, under a great variety of threats to airway function, this production increases. A number of mechanisms may be responsible for augmenting secretion; these include reflexes, initiated by nervous receptors in the airways, and with their efferent limbs in the autonomic nerves to the airways. Cigarette smoke short circuits this reflex pathway; nicotine absorbed from the smoke stimulates ganglion cells directly and so drives secretion. Secretomotor nerves which control the output from submucosal glands include cholinergic, adrenergic and NANC fibres. A number of mediators of the sort released during inflammation and antigen challenge, including prostaglandins and leukotrienes, can also elicit secretion into the airway lumen. Other stimuli act at least partly on unknown mechanisms to release mucus from the surface epithelium into the airway lumen. For example inhalation of dust, which stimulates cough receptors and so acts partly by driving mucus secretion reflexly, also initiates secretion into the denervated airway. Similarly, during the inhalation of cool dry air, the airway lining secretes more mucus, probably by a non-nervous mechanism which responds to drying of the airway surface. More needs to be learned about such apparently direct mechanisms. Different control systems appear to stimulate the production of a variety of mucins from distinct cell types. We do not yet know the physiological and pathological importance of these differences.