Abstract: Objective To investigate the effect of dexamethasone on the expression of muscarinic receptor (MR) mRNA in smooth muscle and infiltration of eosinophils (Eos) in the airway of asthmatic guinea pigs. Methods Thirty healthy guinea pigs were randomized into 3 equal groups, the control group, asthmatic group and dexamethasone therapy group. Asthma was induced in the latter 2 groups with the asthma-inducing agents and received treatments as indicated. Bronchial alveolar lavage fluid(BALF) were collected subsequently from the guinea pigs for examining the total cell number and cell classification, and histopathologic examination of the lung tissue was performed. Semi-quantitative analysis with reverse transcriptional- polymerase chain reaction (RT-PCR) was performed for M₂ and M₃ receptor mRNA in airway smooth muscle. Results Compared with the control and the asthmatic group, the number of Eos in the BALF of dexamethasone therapy group was significantly lower (P<0.01). In spite of the presence of hyperemia and edema in the lung tissues of the dexamethasone therapy group, Eos infiltration was less severe than that in the asthmatic group. As found by RT-PCR, the quantity of M₂ receptor mRNA in the airway smooth muscle of the dexamethasone therapy group was significantly higher than those in both the control and asthmatic groups (P<0.01), and the quantity of M₃ receptor mRNA in the airway smooth muscle of dexamethasone therapy group was significantly higher than that in the asthmatic group, but did not significantly differ from that in the control group. The quantities of M₂ and M₃ receptor mRNAs in the control group were both significantly higher than that in asthmatic group (P<0.01). Conclusion The expression of M₂ receptor is increased in antigen- challenged guinea pigs, and that of M₃ receptor decreased. Dexamethasone can treat asthma by inhibiting inflammatory action involving Eos infiltration, regulating the expressions of M₂ and M₃ receptors and restoring the function of M₂ receptor.