An animal "model" of human lung disease provides a means of testing the effect of irritants and of study in pathogenesis. To relate the model to human lung disease it is necessary to establish the morbid anatomy of the disease. The airway lining system which is under scrutiny here is a. mucus-secreting surface appropriate for the study of chronic bronchitis, the main features of which in man are hypertrophy of mucus-secreting cells, increase in number of goblet cells and extension to the periphery, and an increase in cell activity. The mitotic count is raised and the type of acid glycoprotein modified. These changes can be quantified. They can be reproduced in the experimental animal. The rat is chiefly dealt with here as it has been studied in most detail.