Guinea pigs were exposed 3-5 times/week to bacterial endotoxin by inhalation. The number of free lung cells and the histopathology of lungs and trachea was examined at 5, 10 and 60 weeks. The number of free lung cells was slightly increased but the dramatic response in neutrophils seen after an acute exposure was not present. Histopathological changes, indicative of inflammation were more common in tracheas of exposed animals whereas no difference was found in the lungs.