In the present study, the change in Gq protein level in bronchial smooth muscle of mice with antigen-induced airway hyperresponsiveness (AHR) was determined. BALB/c mice were actively sensitized and repeatedly challenged with ovalbumin antigen to induce bronchial smooth muscle hyperresponsiveness. The contraction induced by 10 μM AlF₄^- (generated by 10 μM AlCl₃ plus 10 mM NaF) of bronchial smooth muscles isolated from the antigen-challenged mice was significantly augmented as compared with that from the control animals. The G αq protein level determined by immunoblotting was also significantly increased in bronchial smooth muscles of the antigen-challenged group. Thus, an upregulation of Gαq protein may be involved in the pathogenesis of bronchial smooth muscle hyperresponsiveness, one of the causes of AHR in asthmatics.