Interstitial fibrosis is a final common pathway for the progression of chronic kidney diseases. Activated fibroblasts have an extremely important role in the progression of renal fibrosis, and transforming growth factor (TGF)-beta1 is a major activator of fibroblasts. Since previous reports indicate that serine protease inhibitors have a potential to inhibit TGF-beta1 signaling in vitro, we hypothesized that a synthetic serine protease inhibitor, camostat mesilate (CM), could slow the progression of renal fibrosis. TGF-beta1 markedly increased the phosphorylation of TGF-beta type I receptor (TbetaIR), ERK 1/2 and smad2/3, and the levels of pro-fibrotic markers, such as alphaSMA, CTGF, and PAI-1, in renal fibroblast (NRK-49F cells), and they were all significantly reduced by CM. In protocol 1, eight-week-old male Sprague-Dawley rats were subjected to unilateral ureteral obstruction (UUO) and were concurrently treated with a slow-release pellet of CM or vehicle for 14 days. Protocol 2 was similar to protocol 1, except that CM was administered 7 days after UUO. CM substantially improved the renal fibrosis that was determined by sirius red staining, collagen expression, and hydroxyproline levels. The phosphorylation of ERK1/2 and smad2/3, and the levels of alphaSMA, CTGF, proMMP-2, and MMP-2 were substantially increased by UUO, and they were all significantly attenuated by CM. These anti-fibrotic effects of CM were also observed in protocol 2. Our current results suggest the possibility that CM might represent a new class of therapeutic drugs for the treatment of renal fibrosis through the suppression of the TGF-beta1 signaling.