To investigate whether the inhibition of muscarinic M(2) receptors results in the enhancement of reflex bronchoconstriction under airway hyperresponsiveness, we evaluated the effects of muscarinic antagonists with or without M(2) antagonist activity on methacholine (MCh)- and SO(2)-induced airway responses in ovalbumin (OVA)-sensitized and -challenged mice. In this model, similar airway hyperresponsiveness to MCh (12 mg/ml) was observed on Days 31 and 37 (2.2-fold and 2.7-fold, respectively). However, airway hyperresponsiveness to SO(2) (0.05 l/min) on Day 37 was less than that on Day 31 (4.0- and 2.7-fold on Days 31 and 37), indicating reflex bronchoconstriction was enhanced on Day 31 in comparison to Day 37. Ipratropium (0.03 - 0.3 mg/ml, inhalation) and Compound A (0.1 - 3 mg/kg, p.o.) inhibited MCh-induced responses on Days 31 and 37. Although ipratropium (0.03 - 1 mg/ml) dose-dependently inhibited SO(2)-induced responses on Day 31, ipratropium at a dose of 0.1 mg/ml significantly increased SO(2)-induced responses on Day 37 (162.2% of the corresponding control). On the other hand, Compound A (0.03 - 0.3 mg/kg, p.o.) inhibited SO(2)-induced responses without any increases on Days 31 and 37. These results suggest that two different conditions of reflex bronchoconstriction are presented in this model: 1) SO(2)-induced responses are enhanced by dysfunctional M(2) receptors on Day 31; 2) the dysfunctional M(2) receptors are partially restored on Day 37. In addition, the inhibition of the restored M(2) receptors further enhance reflex bronchoconstriction.