Abstract: Objective:To investigate the effects of docosahexaenoic acid(DHA) on the atrial fibrosis in atrial fibrillation rats,and mRNA and protein levels of extracellular signal-regulated kinase 1/2(ERK1/2),phosphorylated extracellular signal-regulated kinase 1/2(p-ERK1/2) and type Ⅰ collagen.Methods:Eighty Sprague-Dawley (SD) rats sensitive to acetylcholine-calcium chloride mixture were randomly divided into the control group(CTL group),control treated with DHA group(DHA group),atrial fibrillation group(AF group) and atrial fibrillation treated with DHA group(DHA+AF group) (20 rats each group).The duration of atrial fibrillation in each group was measured.The atrial collagen fiber hyperplasia in rats were detected using Masson staining.The mRNA expression of type Ⅰ collagen in atrial tissue was detected using the quantitative real-time PCR,and the protein levels of ERK1/2,p-ERK1/2 and type Ⅰcollagen in atrial tissue were detected by Western blot.Results:At 10 and 17 days of experiment,the duration of atrial fibrillation in (DHA+AF) group was significantly shorter than that in AF group(P<0.01).Compared with the CTL group,a lot of collagen fibers proliferation in atrial interstitium were found,and the interstitial fibrosis in atrial was higher in AF group.Compared with the AF group, a little collagen fibers proliferation in atrial interstitium was found,and the interstitial fibrosis in atrial was lower in (DHA+AF) group.Compared with the CTL group,the ratio of p-ERK1/2 to ERK1/2 increased in AF group(P<0.05),which in DHA group decreased(P<0.05),and the difference of which between CTL group and(DHA+AF) group was not statistically significant(P>0.05).Compared with the AF group,the ratio of p-ERK1/2 to ERK1/2 decreased in (DHA+AF) group(P<0.05).Compared with the CTL group,the mRNA and protein levels of type Ⅰ collagen in atrial tissue increased in AF group and (DHA+AF) group(P<0.05 to P<0.01).Compared with the AF group,the mRNA and protein levels of type Ⅰ collagen in atrial tissue decreased in(DHA+AF) group(P<0.05).Conclusions:DHA can improve the atrial fibrosis in atrial fibrillation rats,the mechanism of which may be related to the mRNA and protein expressions of type Ⅰcollagen down-regulating by inhibiting the ERK1/2 activation in atrial tissue.