1. Mucous secretion and mucosal permeability by the larynx and trachea, isolated in situ , was investigated in normal rats and in those in which ‘chronic bronchitis' was induced by daily exposure to cigarette smoke for 2 weeks. Fucose was used as a specific marker for the secretion of mucus glycoprotein, and hexose and protein were markers both for mucus and plasma-type glycoproteins present in tissue fluid transudate. Albumin was used as an indicator of the contribution of serum to the secretions. 2. After equilibration, the mean basal secretion of fucose (μg/30 min collection) was significantly higher in ‘bronchitic’ rats than in the controls ( P < 0.01). Mean values for controls were: fucose 3 ( sem 1; n = 9), hexose 41 ( sem 9; n = 8), protein 1082 ( sem 385; n = 8), albumin <2 ( n = 8); for ‘bronchitic’ rats the values were: 24 ( sem 6; n = 7), 101 ( sem 26; n = 8), 2000 ( sem 520; n = 8) and <2 μg ( n = 7) respectively. 3. In control and bronchitic animals acute administrations of cigarette smoke, blown directly through the laryngo-tracheal segment after equilibration, caused significant ( P < 0.05) transient increases in the secretion of fucose, hexose and protein, but not of albumin.