To obtain a detailed pattern of the dynamic evolution and interactions among MMP-13, TIMP-1, type I and III collagen during experimental liver fibrosis.Wistar rats were randomly allocated into a normal group, and a model group. To induce liver fibrosis, rats were intraperitoneally injected with dimethylnitrosamine (DMN) three consecutive times in the first week, then two consecutive times per week, totally for 6 weeks. In the normal control group, rats were treated with saline by the same means. Animals were sacrificed 1, 4, 10, 17, 28, 42, 56 days after starting DMN injections. Conventional histological examinations were performed after hematoxylin and eosin, and Masson stain. Fibrosis stages were classified into 0 to 4. Hydroxyproline contents were determined after liver tissues were hydrolyzed in HCl at 160 degrees C for 2 h and then measured with spectrometry at 560 nm wavelength. mRNA levels of MMP-13, TIMP-1, type I and III collagen were determined by semi-quantitive RT-PCR.In the model group, hepatic type I pro-collagen mRNA expression started to increase on the 10th day after DMN administration (t = 2.85, P < 0.05), type III started to increase on the 28th day (t = 4.16, P< 0.01), and TIMP-1 mRNA expression started to increase on the 4th day (t = 2.60, P < 0.05). They all remained much higher than in the normal group throughout the remaining study period. Hepatic MMP-13 mRNA expression started to increase on the 17th day after DMN administration and remained at a higher level than in the normal group until he 28th day (t = 4.08, P < 0.01), then gradually returned to normal level at the end of the study period.Although hepatic MMP-13 expression transiently increased during liver fibrosis, enhanced expression of TIMP-1 from the early periods of liver fibrosis inhibited the collagen degrading ability of MMP-13, therefore, over-expressed collagen accumulated in the liver. Thus, it is hypothesized that TIMPs play a pivotal role in liver fibrosis.