Objective To investigate the effects of extracellular signal regulated kinase ( ERK)signaling pathway on cell cycle of airway smooth muscle cells( ASMCs) in asthmatic rats. Methods Thirty Wistar rats were randomly assigned to a control group and an asthma group( 15 rats in each group) . Asthma model was established by ovalbumim sensitization and challenge. ASMC were isolated and cultured in vitro. The ASMCs from the asthmatic rats were treated with ERK activator epidermal growth factor ( EGF)and inhibitor PD98059, respectively. The expressions of cyclin D1 and CDK2 in ASMCs were detected by immunocytochemical staining. The expressions of ERK1 /2 and p-ERK1 /2 protein were observed by western blotting for measurement of ERK activation rate. Results Compared with the control group[ 54. 17 ±6. 11,61. 04 ±4. 09, ( 49. 91 ±3. 26) % , respectively] , the expressions of cyclin D1 protein and CDK2 protein,and the rate of ERK activation of ASMCs from the asthmatic rats significantly increased[ 76. 15 ±4. 88,92. 30 ±7. 95, ( 82. 37 ±5. 78) % , respectively] ( P lt; 0. 05) . Furthermore, compared with those before treatment, the expression of cyclin D1 and CDK2, and the rate of ERK activation of ASMCs significantly decreased after treatment with PD98059 [ 58. 78 ±4. 60, 69. 15 ±5. 83, ( 54. 01 ±4. 12) % , respectively]( P lt; 0. 05) , and significantly increased after treatment with EGF[ 119. 28 ±8. 14, 134. 77 ±9. 26, ( 91. 57 ±5. 32) %, respectively] ( P lt;0. 05) . Conclusion ERK1/ 2 participates in proliferation regulation of ASMCs in asthma by enhancing the expressions of cyclin D1 and CDK2, which promotes quiescent cells into S phase.