Using beagle dogs, we have developed an animal model for the evaluation of the effect of chronic cigarette smoking on pulmonary defense and function and on lung structure. The smoking apparatus developed allows the dog actively to inhale properly diluted (1:4) smoke directly from the cigarette through a mouthpipe. In this model, 6-month and 1-year periods of mild to moderate smoking caused impairment of tracheal mucociliary transport and bacteriosuppressive activity of alveolar macrophages, with little change in pulmonary function. Morphologically, subtle, but significant, lesions were noted in the central airways and bronchiolar walls, consisting of tracheal epithelial basal cell hyperplasia, proliferation of goblet cells in central airways, and peribronchiolar infiltration by inflammatory cells. Morphometry of bronchiolar size distribution, volume proportion of parenchymal structure, and alveolar surface area, however, failed to show significant differences between the groups.