We studied whether airway hyperresponsiveness induced by ozone exposure is associated with changes in the numbers of different types of cells in bronchoalveolar lavage in dogs. Airway responsiveness to acetylcholine and the numbers of cells in lavage fluid were determined 1 wk before and then 1 h and 1 wk after 2-h exposures to filtered air and to ozone (3.0 ppm) in each of 5 dogs. Airway responsiveness and the numbers of cells in lavage fluid did not change after exposure to filtered air. By contrast, airway responsiveness increased markedly 1 h after exposure to ozone and returned to control levels 1 wk later. In addition, the numbers of neutrophils and of ciliated epithelial cells in lavage increased markedly 1 h after ozone and returned to control levels 1 wk later. Our previous study showed that airway hyperresponsiveness induced by ozone is associated with an influx of neutrophils into the most central airways (1); the present results suggest that the hyperresponsiveness is also accompanied by an influx of neutrophils into more distal airways and by desquamation of airway epithelial cells.