Hyperoxia and severe hypoxia are known to depress tracheal mucus flow in vivo. It is not clear, however, whether this is also seen in bronchial mucociliary transport system. The author attempted to ascertain acute effects of hyperoxia and moderate hypoxia on bronchial mucociliary clearance by analyzing the regional clearance of aerosolized radioactive tracers within the lung. Eleven healthy persons were exposed to pure oxygen or moderate hypoxia (mean end-tidal PaO2 57.5 mmHg) for 30 min. Twenty four patients with chronic pulmonary emphysema were studied for the chronic effect of hypoxemia on regional mucociliary clearance. They had slight hypoxemia (mean PaO2 76 mmHg). After inhalation of 99mTc-albumin aerosols, clearance of deposited aerosols was quantified as a function of time. The results were analyzed for whole right lung in the acute hyperoxic and hypoxic studies, and for 3 concentric areas representing central, mid, and peripheral regions of the right lung in the study of patients. In healthy subjects, breathing pure oxygen caused significant depression that started 30 min after the initiation of oxygen exposure and was kept up even after stopping the exposure. The clearance was significantly impaired during exposure to moderate hypoxia, though it seemed to be transient. The patients with chronic pulmonary emphysema had a significantly lower clearance in the central region than that in asymptomatic smokers (p less than 0.01). There was no significant correlation, however, between the degree of hypoxemia and the regional clearance. These results suggest that 1) acute exposure to pure oxygen and moderate hypoxia causes bronchial mucociliary dysfunction in humans, 2) the patients with chronic pulmonary emphysema have a lower clearance in the central region of the lung than asymptomatic smokers, and 3) chronic slight hypoxemia has no apparent effect on bronchial mucociliary clearance.