OBJECTIVE: To explore the imbalance between the expression of metalloproteinases (MMPs) and that of tissue type inhibitors of metalloproteinase (TIMPs) in the renal tubulointerstitial lesions of aging rats and the potential role of this imbalance. METHODS: Forty-eight male 26-month-old Wistar rats were randomly divided into 2 groups of 24 rats: unilateral ureteral obstruction (UUO) group with the left ureter ligated and excised and false operation group used as control group. Forty-eight male 3-month-old Wistar rats were randomly divided into 2 groups of 24 rats: UUO group and false operation group just as in the 26-month-old rats. Every group was randomly divided into 3 subgroups of 8 rats: 3-day group, 7-day group, and 14-day group to be killed 2, 7, and 14 days after the operation respectively. The 2 kidneys of each rat were taken out. Routine pathological examination and immunofluorescence (IF) examination were made to calculate the area of renal tubulointerstitial fibrosis and the expression of IV type collagen. RT-PCR and Western blotting were used to detect the expressions of mRNA and protein of TIMP-1, TIMP-2, MMP-2, and MMP-9 in the kidney at different time points. Gelatin zymography was used to detect the proteolytic activity of MMP-2 and MMP-9. RESULTS: The renal interstitial lesion was more significantly in the 26-month-old UUO rats than in the 3-month-old UUO rats since the 3rd day after operation. Both the expression of MMP-2 mRNA and the expression of MMP-9 mRNA were not different between the 2 control groups. In the control groups, TIMP-1 and TIMP-2 Both MMP-2 mRNA and MMP-9 mRNA were expressed in both control groups, without significant differences between these 2 control groups. TIMP-1 mRNA and TIMP-2 mRNA were only weakly expressed in the renal tissues of the 3-month-old control group, however, the expressions of both TIMP-1 and TIMP-2 were significantly stronger in the 16-month-old control group than in the 3-month-old control group. The expressions of TIMP-1 mRNA and TIMP-2 mRNA at different time points were significantly stronger in the obstructed side than in the healthy side in the UUO groups, and significantly stronger in the 26-month-old rats than in the 3-month-old rats. MMP-2 protein and MMP-9 protein were expressed in the 3-month-old and 26-month-old controls without difference between them. The expressions of MMP-2 protein and MMP-9 protein at different time points were significantly stronger in the obstructed side than in the healthy side in the UUO groups, without significant difference between the 26-month-old UUO rats and the 3-month-old UUO rats. The expressions of TIMP-1 protein and TIMP-2 protein were very weak at different time points in the renal tissues of the 3-month-old controls and were significantly stronger in the 26-month-old control rats. In the UUO rats the expressions of TIMP-1 protein and TIMP-2 protein in the renal tissues of the obstructed side at different time points were all significantly stronger for both age groups in comparison with the control groups of the same age and were significantly stronger in the 26-month-old UUO rats than in the 3-month-old UUO rats without significant difference between the 26-month-old UUO rats and the 3-month-old UUO rats. The MMP-2 activity and MMP-9 activity of the 26-month-old rats were both significantly lower than those of the 3-month-old control rats. The MMP-2 activity and MMP-9 activity at different time points of the 2 UUO groups were all significantly lower than those of the control groups of the same age, and those of the 26-month-old were significantly lower than those of the 3-month-old rats The MMP-2 activity and MMP-9 activity were negatively correlated with the expressions of TIMP-2 and TIMP-1. CONCLUSION: The abnormal expression of MMPs/TIMPs including higher expression of TIMPs and decreased proteolytic activity of MMPs induced by aging may be one of the factors aggravating the renal tubulointerstitial lesions of aging rats.