Hypersecretion is a feature of chronic bronchitis and other airway diseases, often with ensuing complications of infection, failure to clear secretions, airflow obstruction and progressive disability leading to premature death. The histological counterpart is an increase in mucus-secreting tissues ie. hyperplasia of mucous cells in the surface epithelium and hypertrophy of submucosal glands. The former change, associated with an extension of mucous cells into peripheral airways from which mucus is difficult to clear by cough, is of particular functional significance. In the studies presented herein the laboratory rat, free of respiratory infection, has been used to study the pathogenesis of epithelial mucous cell hyperplasia induced by 2 weeks exposure to an atmosphere of tobacco smoke (TS).