Following the demonstration of morphologic changes induced by a sulphur(IV) aerosol inthe extrapulmonary airways (Takenaka et al., 1990), the pulmonary airways were examined in order to evaluate pulmonary effects. After continuous exposure for 290 days to a respirable sulphur(IV) aerosol at a concentrationequivalent to a sulphur dioxide concentration of 0.6 mg m-3, lung samples from 8 exposed and 3 unexposed beagle dogs were prepared according to the modified cascade sampling and vertical section methods, and bronchial glands and alveolar regions were analyzed morphometrically. No differences were observed in the ratio of bronchial glands to bronchial walls between the exposure and control groups. However, the exposure group showed a slight decrease in the volume density of the septum. This decrease resulted in a reduction in the septal surface area, indicating enlargement of the alveolar air space. One of 8 exposed dogs showed clear morphologic evidence of focal dilation of the alveoli and alveolar sac. There was a significant decrease in the calculated diffusing capacity in the exposure group. These findings indicate that long-term exposure to airborne sulphur(IV) has adverseeffects on both the upper and lower parts of the respiratory system of beagle dogs.