Parasympathetic nerves play an important role in modulating smooth muscle tone and mucus secretion in the airways. This modulation can occur through a variety of afferent inputs, from the central nervous system, at parasympathetic ganglia, and on post-ganglionic efferent fibres themselves. Abnormalities at any of these sites could, and in some patients probably do, contribute to the abnormalities in smooth muscle contraction and mucus secretion that characterize a number of airway diseases, including asthma. Because these pathways play little or no role in the exaggerated bronchoconstrictor responses in many patients, however, an abnormality of the parasympathetic pathway cannot be the sole explanation for abnormal airway hyperresponsiveness.