To test the hypothesis that the development of airway hyperresponsiveness (AHR) lasting greater than or equal to 3 days after the last antigenic exposure required repeated mediator release, we compared dose-response changes in lung resistance (RL) to acetylcholine (ACh) in animals sensitized with 1% ovalbumin (OA), 4% Bordatella pertussis aerosol and subsequently challenged with 0.5% OA aerosol twice weekly for 4-6 wk vs. animals receiving saline aerosol instead of OA. Despite antihistamine pretreatment, each OA challenge produced cyanosis and inspiratory indrawing. Blood gas analysis in six guinea pigs revealed an immediate fall in arterial PO2 (PaO2) from 104.3 +/- 4.9 to 35.4 +/- 2.2 Torr after a 1-min exposure to aerosolized OA. ACh dose-response measurements of RL 3 days after the last OA challenge demonstrated a leftward shift and an increased magnitude of response. These differences were less marked at 7 days, and by 14 days after the last OA challenge, ACh dose-response curves were not different from those of control guinea pigs. Sensitization without repeated antigen challenge did not cause hyperresponsiveness. Morphometric analysis showed significantly increased numbers of eosinophils in the epithelium of airways in hyperresponsive guinea pigs, without neutrophil infiltration or alterations in epithelium and airway wall areas. We conclude that repeated antigenic challenge, but not sensitization alone, causes prolonged AHR in guinea pigs, which is associated with tissue eosinophilia.