OBJECTIVE: Metabolic syndrome is characterized by abdominal obesity, hypertriglyceridemia and hyperglycemia. Fatty acid binding protein 4 (FABP4), as a member of intracellular lipid chaperones, is not only engaged in lipid transport but involved in inflammation and insulin resistance. The present study was to investigate the effects of BMS309403, a specific FABP4 inhibitor, on metabolic syndrome and its possible molecular mechanisms in islets. MATERIALS AND METHODS: Leptin receptor knockout (Lepr-/-) rat, a novel and representative animal model of metabolic syndrome, was adopted in this study. Lepr-/- male rats and their wild littermates were grouped and intragastrically administered with BMS309403. Glucose Tolerance Test (GTT) and Insulin Tolerance Test (ITT) were performed on all rats. Serum insulin was detected by ELISA. The metabolic characters, as well as liver and kidney functions, were evaluated by serum biochemical assay. Immunohistochemistry and Western blot were adopted to detect the expression levels of FABP4, CD68, GRP78, ATF6, p-IRE1a, and Cleaved caspase-3. RESULTS: Lepr-/- rats showed prominent characteristics of metabolic syndrome with increased FABP4, inflammatory infiltration, ER stress and apoptosis in islets. BMS309403 administration attenuated inflammation, ER stress and apoptosis in Lepr-/- rat islets while stimulating insulin secretion as well as improving manifestation of metabolic syndrome without hepatic and renal toxicity. CONCLUSIONS: FABP4 increased in Lepr-/- rat islets and might be involved in the regulation of islet inflammation and apoptosis via ER stress. FABP4 inhibitor BMS309403 could ameliorate islet inflammation and apoptosis in metabolic syndrome through suppressing ER stress.