Bronchial hyperresponsiveness (BHR) is a major characteristic of bronchial asthma. The pathogenesis of BHR remains to be fully elucidated, but is considered to be closely linked to airway inflammation. Animal models might provide us with useful data for a better understanding of the interrelationship between these phenomena. In the present study we investigated the effect of a single and chronic exposure to inhaled antigen on bronchial responsiveness and airway morphology in actively sensitized Brown Norway rats. Immunization to ovalbumin (OA) did not cause airway inflammation, but induced a small, transient decrease in bronchial responsiveness to 5-hydroxytryptamine (5HT) on Day 10, which returned to baseline on Day 16. By 24 h after a single exposure to aerosolized OA, a significant decrease in the provocative concentration of 5HT causing a 50% increase in lung resistance (PC50RL 5HT) was observed, compared with immunized, saline-exposed animals (7.7 +/- 0.8 versus 10.8 +/- 1.0 micrograms/kg). This was accompanied by the influx of neutrophils and few eosinophils in bronchoalveolar lavage fluid. Repeated daily or intermittent exposure to aerosolized OA enhanced airway inflammation, characterized by the presence of neutrophils, eosinophils, and lymphocytes in bronchoalveolar lavage fluid. Histologic analysis revealed patchy inflammatory infiltrates, located predominantly around bronchi and bronchioli. Despite these inflammatory changes, bronchial responsiveness was not significantly different from that of control animals. We therefore conclude that the induction of airway inflammation is not always associated with BHR.