Objective:This study was aimed to investigate the effects of doxycycline on the activity and mRNA expression of MMP-9 and TIMP-1 in CPB in a rat model. Methods:Male Sprague-Dawley rats were randomly divided into 3 groups (n = 12,respectively):the sham group,the CPB group,and the CPB+Dox group. The rats were executed at the termination of CPB (T1),6h after termination of CPB(T2),respectively. BALF and lung tissues were harvested at T1 and T2. The activity of MMP-9 and TIMP-1 was detected by Western-blot analysis. The expression of MMP-9 and TIMP-1 in lung tissue was detected by RT-PCR. Results:The edema of lung tissue was significantly reduced in the CPB+Dox group,and neutrophils were decreased in the CPB group. The activity of MMP-9 in the CPB+Dox group decreased significantly compared with that in the CPB group and the highest expression occured after 6 h. The activity of TIMP-1 had a weaker increasing trend in the CPB and the CPB+Dox group after the ending of CPB,and the expression difference was not obvious in the response time of the two groups. The expression of MMP-9 mRNA was significantly increased after CPB in the CPB and the CPB+Dox group,but significantly decreased at T2 time point in the CPB+Dox group. The ratio of MMP-9/TIMP-1 was significantly increased in the CPB and CPB+Dox group,but the ratio in CPB+Dox group was significantly lower than that of CPB group at T2 time. Conclusion:CPB could induce the increasing of the activity and mRNA of MMP-9 of lung,causing the ratio of MMP-9/TIMP-1 imbalanced severely at the early postoperative. Doxycycline could inhibit the expression of MMP-9 activity and mRNA in rat lung tissue after CPB,and may increase the expression of TIMP-1 in rats after CPB,and inhibit the expression of MMP-9 mRNA indirectly,which could improve the ratio of MMP-9/TIMP-1 imbalance.