Histopathologic evaluation was made of lungs of guinea pigs exposed by inhalation to cotton dust for 3, 6, and 12 months. Changes were observed near the end airways. These consisted of type II cell hyperplasia and thickening of alveolar walls around the alveolar ducts. After one year of exposure, hyperplasia of the bronchiolar epithelium was apparent. There was no fibroplasia or collagen deposition in the lungs of any of the exposed animals. In addition, no significant changes were found in the nasal cavities. These findings of small airway changes are consistent with those which have been reported in workers with byssinosis.