The secretion of the bronchial submucosal glands is important both in the normal function of the respiratory tract and as a complicating factor in chronic lung disease. Bronchial secretion is a mixture of glycoproteins, proteins and transudate* The unique physico-chemical properties of the secretion are due principally to the glycoproteins produced by mucous and serous cells in the bronchial glands and by goblet cells of the surface epithelium. In diseaset the bronchial glands often show hypersecretion and hypertrophy, accompanied by changes in the composition, and thus the physical characteristics, of their secretion* The complexity of these functions and their inter-relationship demands the use of a variety of techniques in their investigation* first, the arrangement of the secretory cells in the human bronchial glands has been established and the presence of a functional duct system revealed as an essential part of the gland. Second, the use of organ culture with the addition of drugs has allowed investigation of the control of the secretion of each cell type to be studied independently. The level of gland secretion is related to the degree of hypertrophy as is also the gland's sensitivity to parasympathetic drugs. Third, an animal 'model' of bronchial gland hypertrophy and hypersecretion has been produced by the in vivo use of drugs. For the first time changes in the tracheo-bronchla1 glands have been found after the administration of isoprenaline and pilocarpine. Fourth, the rheological behaviour of human bronchial secretion in various diseases has been related to its chemical composition. A viscosity pattern not previously described for any biological fluid has been found to be characteristic of sputum. Finally, in cystic fibrosis, no abnormality has been found in gland secretion rate, drug; response or viscosity levels, although a variant in the viscosity pattern has been detected.