To better understand the mechanism of ozone-induced airway hyperresponsiveness we determined the time course of the ozone effect in dogs. To do this we assessed airway responsiveness before ozone exposure and then at 1 h, 1 day, and 1 wk after ozone exposure. To assess responsiveness we anesthetized the dogs and obtained dose-response curves of increasing concentrations of acetylcholine or histamine aerosols delivered to the airways vs. pulmonary resistance. Ozone exposures were carried out with the dogs awake and at rest in an exposure chamber for 2 h breathing either through the nose and mouth at a level of 2.2 ppm or through a tracheostomy at a level of 1.0 ppm. For both acetylcholine and histamine and for both routes of ozone delivery airway responsiveness increased most markedly at 1 h after ozone, increased to a lesser degree 1 day later, and returned to control levels by 1 wk. The results are similar to our previous studies in humans that showed that ozone-induced hyperresponsiveness occurs shortly after exposure and is rapidly reversible and suggest that the ozone effect is linked to an acute inflammatory response in the airways.