Objective To explore the possible mechanism of Yishen Huayu Decoction（ 益肾化瘀方） in treating renal interstitial fibrosis（ RIF）. Methods Totally 126 SD rats were randomly divided into sham operation group

model group

valsartan group

high

medium and low dose Yishen Huayu Decoction group

with 21 rats in each group. Rats in other groups were established RIF models by unilateral ureter ligation method except the sham operation group. From the second day of modeling

the sham operation group and the model group were given normal saline20 ml /（ kg·d）. The high

medium and low dose Yishen Huayu Decoction groups were given Yishen Huayu Decoction in concentration 32

16 and 8 g /（ kg·d） respectively. The valsartan group was given valsartan capsule solution 13. 3mg /（ kg·d）

gavaged once every day

for 28 days. On the 7th

14 th and 28 th day after operation（ 7 rats were examined at each time point）

pathological changes of renal interstitial were examined under optic microscope. Renal interstitial fibrosis was scored. The expressions of matrix metalloproteinase-1（ MMP-1） mRNA and tissue inhibitor matrix metalloproteinase-1（ TIMP-1） mRNA in rats’ nephridial tissue were detected. Results Under optic microscope

HE stain and Masson stain showed that 7 days after modelling

rats showed renal interstitial edema. Interstitial inflammatory corpuscles showed focal infiltration. A large number of renal tubular atrophied or disappeard until the28 th day. The fibrosis degree showed progressive aggravation. Inflammatory corpuscles infiltration reduced in all treated groups. The fibrosis degree in the treated groups relieved by varying degrees compared with that in the model group.The pathological changes in the high dose Yishen Huayu Decoction group were similar to those in the valsartan group.Compared with the sham operation group at the same time point

the RIF scores and the expressions of MMP-1mRNA and TIMP-1mRNA in nephridial tissue of rats in other groups all increased on the 14 th and 28 th day（ P < 0. 05 or P < 0. 01）. Compared with the model group at the same time point

the RIF scores in the high

medium and low dose Yishen Huayu Decoction groups and the valsartan group decreased significantly on the 14 th and 28 th day. The expression of MMP-1mRNA in the high dose Yishen Huayu Decoction group continued to increase on the 28 th day. While the expression of TIMP-1mRNA decreased on the 7th

14 th and 28 th day（ P < 0. 05 or P < 0. 01）. Conclusion Yishen Huayu Decoction might correct MMP-1 / TIMP-1 imbalance by inhibiting the expression of TIMP-1 in nephridial tissue and showed the effect of resisting RIF.