Objective: To investigate the role of cyclin kinase inhibitor P27 on the proliferation of mesangial cell(MC) in Thy1 glomerulonephritis(GN) rats. Methods: The Thy1 model of experimental mesangial proliferative glomerulonephritis was induced by intravenous injection of rabbit anti-thymocyte plasma. The P27 protein of glomerular lysate was detected with Western blotting analysis. The P27 mRNA was determined with RT-PCR. The extracellular matrix (ECM) protein (fibronectin and type Ⅳ collagen) of glomerular lysate was examined with ELISA. Results: Glomerular P27 protein of Thy1 model rat decreased compared with that of normal rat. The level of P27 was associated with the degree of MC proliferation:P27 began to decrease on day 1 in Thy1 model, then reached the lowest on day 3 when MC proliferation was maximal, then returned to 60% of the baseline level on day 5 when MC proliferation began to resolute. Glomeruar P27 mRNA remained constant in Thy1 GN(3 d) rats. Glomerular fibronectin(Fn) and type Ⅳ collagen increased in Thy1 model: They reached the highest on day 3(P