Matrix metalloproteinase (MMP)-9 plays an important role in the pathogenesis of colitis. Recent studies have demonstrated that chymase is involved in the conversion of promatrix metalloproteinase (proMMP)-9 to MMP-9. However, whether chymase contributes to the activation of proMMP-9 in colitis has remained unclear. In this study, we administered 5% dextran sodium sulfate (DSS) solution to mice for 7 days. At 7 days after starting administration, both chymase activity and MMP-9 activity were significantly increased. In extract from colitis in DSS-treated mice, MMP-9 activity was significantly increased after 8 h of incubation, but increased activity was almost completely suppressed in the presence of a chymase inhibitor, 2-(5-formylamino-6-oxo-2-phenyl-1,6-dihydropyrimidine-1-yl)-N-[{3,4-dioxo-1-phenyl-7-(2-pyridyloxy)}-2-heptyl] acetamide (NK3201). At 7 days after starting administration, intestinal length was significantly shorter in DSStreated mice than in normal mice, but these changes were significantly prevented by NK3201 (10 mg/kg per day i.p.). Disease activity index and histological damage score were also significantly reduced by NK3201. The filtrated neutrophil number was significantly decreased by NK3201. Furthermore, NK3201 significantly attenuated not only chymase activity but also MMP-9 activity in DSS-treated mice. These findings suggest that chymase plays an important role in the development of colitis via MMP-9 activation.Ulcerative colitis is an inflammatory bowel disease (IBD) affecting the distal colon; the etiology remains unclear. In chronic tissues of patients with IBD, the synthesis and release of inflammatory cytokines and recruitment of inflammatory cells induce the degradation of extracellular matrix (ECM). Matrix metalloproteinases (MMPs) are important enzymes for degrading ECM proteins, and MMPs are reportedly involved in the pathogenesis of IBD via ECM degradation . MMPs are a family of zinc-and calcium-dependent endopeptidases. Among the MMPs, MMP-2 and MMP-9 are known as gelatinases and are consistently up-regulated during active flares of IBD in patients and animal models of colitis (Baugh et al., 1999;Tarlton et al., 2000). In particular, MMP-9 may play an important role in the degradation of ECM in ulcerative colitis. Oral administration of dextran sodium sulfate (DSS) is used to induce colitis in animal models, and the resulting colonic lesions resemble those observed in patients with ulcerative colitis (Elson et al., 1995). DSS-induced colitis is significantly attenuated in MMP-9-deficient mice but not in MMP-2-deficient mice (Castaneda et al., 2005;Garg et al., 2006). Thus, MMP-9 inhibition might offer a useful strategy for attenuating the development of colitis.Chymase is a chymotrypsin-like serine protease located in the secretory granules of mast cells. Serine proteases, such as MMP-3, and trypsin are known to process promatrix metalloproteinase (proMMP)-9 to MMP-9 in vitro (Sang et al., 1995;Shapiro et al., 1995). Fang et al. (1996Fang et al. ( , 1997 reported tha...