The effects of chronic exposure to ovalbumin (OA) aerosol were studied in Brown Norway rats following intraperitoneal injections with OA and AI(OH)3 and exposure to OA or saline aerosols, once or every third day for 3 to 8 wk. Measurements of airway responsiveness to acetylcholine (ACh) aerosol at 18 to 24 h after allergen exposure showed a significant increase in -logPC150, the concentration of ACh needed to cause a 150% increase in baseline lung resistance, in animals single-exposed or chronic OA-exposed for 3 wk, compared with saline-exposed control animals. The group receiving 8 wk of OA exposure demonstrated no difference from the control animals with -logPC150 lower than that of the two previous groups (p < 0.001). In all three groups, BAL fluid showed a significant increase in neutrophils, but a significant increase in eosinophils (p < 0.01) was only observed in the single-exposed group when compared with saline-exposed control animals. In the 8-wk exposed rats, there was a higher recovery of macrophages and lymphocytes (p < 0.01) compared with control animals and the other two groups. AHR, present after single or 3-wk repeated exposure, disappears by 8 wk of continuous allergen exposure. Both the enhancement and suppression of AHR may be linked to OA-induced immune and inflammatory mechanisms.