JTT-501, 4-[4-[2-(5-methyl-2-phenyl-4-oxazolyl)ethoxy]benzyl]-3,5-isoxaz olidinedione, is an isoxazolidinedione derivative which is structurally distinct from thiazolidinediones such as pioglitazone and troglitazone. We investigated the effects of JTT-501 on insulin-sensitizing activity and in rodent diabetic models. JTT-501 enhanced insulin-stimulated cell differentiation of 3T3-L1 fibroblasts with an EC50 value of 110 nM. Furthermore, JTT-501 activated peroxisome proliferator-activated (PPA) gamma and alpha receptors with the EC5-fold values of 0.28 and 5.4 microM, respectively. In the non-insulin-dependent diabetes mellitus model KK-Ay mice, JTT-501 improved hyperglycemia, hyperinsulinemia and hypertriglyceridemia, and enhanced insulin-stimulated glucose oxidation in adipose tissues. JTT-501 was also effective in the non-insulin-dependent diabetes mellitus model Zucker diabetic fatty (ZDF) rats but not in the insulin-dependent diabetes mellitus model streptozotocin-induced diabetic mice. These observations suggest that JTT-501 enhances insulin sensitivity in peripheral tissues and improves hyperglycemia, hyperinsulinemia, and hypertriglyceridemia in non-insulin dependent diabetes mellitus models. In particular, the triglyceride-lowering activity of JTT-501 is a unique characteristic compared to the thiazolidinediones. Therefore, JTT-501 may be a promising antidiabetic agent for treating non-insulin-dependent diabetes mellitus patients with insulin resistance.