Hypoglycemic mechanism of brazilin was investigated in the streptozotocin induced diabetic rats. Plasma glucose levels of diabetic rats were significantly ame]iorated by the treatment of brazilin, but there were no changes in plasma insulin levels. Brazilin increased insulin binding capacity to epididymal adipocytes, and Scatchard analysis revealed that this increase in insulin binding was not due to the increase of insulin binding sites but that of binding affinity. 2-Deoxyglucose uptake of epididymal adipocytes was significantly augmented by the intraperitoneal administration of brazilin and the same result was obtained in in vitro study. Glucose oxidation and lipogenesis in epididymal adipocytes were significantly enhanced by the treatment of bracilin.