A mathematical model of the airways is developed which focuses on the dynamic exchange characteristics of heat, water and soluble gas. A typical airway segment is divided radially into three regions: the airway lumen, a thin mucous layer of variable thickness coating the airway wall, and an underlying nonperfused tissue layer. A bronchial circulation capillary bed lies beyond the nonperfused tissue layer. The simultaneous exchange of water, heat and soluble gas is dealt with using the model of Tsu et al. (Ann. Biomed. Eng. 16:547-571, 1988). In the case of excretion of ingested ethyl alcohol from the bronchial and pulmonary circulations, the model predicts that during inspiration, because of the alcohol flux from the airway mucosa, a concentration of alcohol in equilibrium with mucus is achieved in the inspired air before the respiratory bronchioles are reached. During exhalation, much of this alcohol redeposits on the airway surface. The net flux of alcohol from the airway surface exceeds the flux of alcohol from the mouth in the exhaled gas indicating that the exhaled alcohol comes from the airways and bronchial circulation rather than from the alveoli and the pulmonary circulation. Alcohol flux moves farther into the airways with oral breathing compared to nasal breathing. Increased ventilation shifts the alcohol flux more alveolarward. Changes in inspired air temperature and humidity have almost no effect on the distribution of alcohol flux in the airways.