Previous studies in a rat model of Sephadex induced lung inflammation showed that 4-Thiouridine (4SU), a thiol substituted nucleoside, was very effective in reducing edema, leukocyte influx and TNF levels in bronchoalvelolar lavage fluid. However, little is known about the factors and mechanisms underlying these effects. In the present study, we have used two separate mouse models of chronic inflammation, a model of dextran sulphate sodium (DSS) induced colitis and a model of antigen induced arthritis, to evaluate the anti-inflammatory effect of 4-thiouridine. We have analyzed a broad spectrum of inflammatory mediators in order to delineate the mechanisms behind a potential anti-inflammatory effect of 4SU. Colitis was induced in C57BL/6 mice by administration of 3.5% DSS in drinking water for 5 days and the potential anti-colitic effect of 4SU was assessed by monitoring the disease activity index (DAI), measurement of colon length and histopathological analysis of colon tissue. We analyzed tissue myeloperoxidase (MPO) activity, serum pro-inflammatory cytokines (IL-1 beta, IL-6 and TNF), mRNA and protein expression of pro-inflammatory cytokines, COX-2, and NF-kappa B activity in colitis tissue. Intracolonic administration of 4SU (5 mg/kg & 10 mg/kg.) significantly inhibited MPO activity and reduced the levels of pro-inflammatory cytokines (IL-1 beta, IL-6 and TNF) as well as COX-2. Further, NF-kappa B activation was also blocked by attenuating the phosphorylation of IkB kinase (IKK alpha/beta) in DSS-induced colitis tissues. Arthritis was induced by intra-articular injection of mBSA in the knee of NMRI mice pre-immunized with mBSA and 4SU was administered locally by direct injection into the knee joint. The antiarthritic potential of 4SU was calculated by histopathological scores and histochemical analysis of joint tissue. Further, immunohistochemistry was used to study inflammatory cell infiltration and expression of cytokines and adhesion molecules in the synovium. Local administration of 50-100 mg/kg 4SU at the time of arthritis onset clearly prevented development of joint inflammation and efficiently inhibited synovial expression of CD18, local cytokine production and recruitment of leukocytes to the synovium. Taken together, our data clearly demonstrates a potent anti-inflammatory effect of 4SU in two experimental models. In conclusion 4SU could be a new promising candidate for therapeutic modulation of chronic inflammatory diseases like ulcerative colitis and arthritis.