Objective To investigate the effects of smoking intensity, duration and cessation on mRNA and protein expressions of matrix metalloproteinase-9 ( MMP-9) in tracheal epitheliumof rats, and the relationship between smoking or smoking cessation and airway remodeling in chronic obstructive pulmonary disease ( COPD) . Methods Forty Wistar rats were randomly divided into 5 groups, ie. a normal control group, a long termheavy smoking group, a short termheavy smoking group, a long termlight smoking group,and a smoking cessation group which was exposed to room air for 10 weeks after long term heavy smoking.The expressions of MMP-9 mRNA and protein in tracheal epithelium of rats were detected by in situ hybridization and munohistochemistry respectively. Results ( 1) The pathological changes of emphysema were observed in the lung tissue of every smoking rat, and were most sever in the long term heavy smoking group. ( 2) Compared with the normal control group [ ( 0. 88 ±0. 88) PU, ( 2. 80 ±1. 66) PU] , the expressions of MMP-9 mRNA and proteins in tracheal epithelium were remarkable elevated in the long term heavy smoking group [ ( 22. 01 ±2. 86) PU, ( 20. 81 ±2. 46) PU] , the short term heavy smoking group [ ( 14. 94 ±3. 46) PU, ( 13. 68 ±2. 00) PU] , the long term light smoking group [ ( 6. 92 ±2. 71) PU,( 8. 84 ±1. 80) PU] and the smoking cessation group [ ( 19. 00 ±3. 36) PU, ( 14. 82 ±1. 74) PU] ( P lt;0. 01) . Compared with the long term heavy smoking group, the expressions of MMP-9 in tracheal epithelium were decreased in other three smoking groups ( P lt; 0. 05) . Conclusions Smoking could increase the expression of MMP-9 in tracheal epithelium and cause trachea damage and remodeling with intensity and duration in rats. Smoking cessation could decrease the MMP-9 expression and alleviate trachea remodeling,suggesting its role in the prevention of COPD.