We examined the relationship between ozone (O3)-induced airway hyperresponsiveness (AHR) and inflammation in guinea pigs. Inhalation of methacholine (MCh) was adopted in the time course study of AHR that was assessed by measuring pulmonary inflation pressure after O3 exposure (3 ppm, for 2 hr) because the degree of AHR detected by inhalation of MCh was greater than that detected by i.v. administration. AHR was detected up to 5 hr after O3 exposure and was not observed at 24 and 48 hr. In the bronchoalveolar lavage (BAL) study, the numbers of neutrophils, eosinophils, lymphocytes and macrophages in BAL fluid (BALF) reached maximum at 24 hr or later. On the other hand, the number of airway epithelial cells in the BALF significantly increased at 2 and 5 hr. In the histological study, disorder and impairment of the airway epithelium in the trachea and lung were observed at 2 and 5 hr. Changes in the airway epithelium were recovered at 48 hr, although an increase in leukocytes was observed in the lung. These results indicate that O3-induced AHR in guinea pigs is most probably associated with impairment of the epithelium rather than with infiltration of inflammatory cells in the airway.