Lungs of guinea pigs exposed subchronically to cotton dust at 1.6 mg/m3 were evaluated using histological and morphometric techniques. Histopathologic examination of lungs including major bronchi, numerous small bronchioles, trachea, nasal cavity and peribronchial lymph nodes, failed to reveal any notable difference between exposed and control guinea pigs. Using electron microscopy, increased numbers of alveolar macrophages and Type II cells were detected in exposed animals. Focal alveolar wall thickening was also observed. There was also significant slight thickening of bronchiolar smooth muscle. These changes were not of sufficient severity to have affected diffusion capacity.