We have previously isolated spontaneous insulin-resistant mice (ddY-H) and non-insulin-resistant mice (ddY-L) from ddY mice. In the present study, age-dependent onset of insulin resistance in obese ddY-H mice was investigated by comparing with lean ddY-L mice. In ddY-H mice fed a standard diet, an increase in elevation of glucose-stimulated plasma insulin level, glucose intolerance in an intraperitoneal glucose tolerance test, and a reduction of hypoglycemic action of insulin were found at 9 weeks of age, but not at 6 weeks of age. When ddY-H mice were administered nateglinide, a greater elevation of plasma insulin level and a less decrease of serum glucose level were observed at 9 weeks of age. These changes developed progressively with age. These findings suggest that insulin resistance is induced at 9 weeks of age. The age-related change in insulin resistance was correlated with reductions in mRNA expression and protein content of the insulin receptor (InsR), and insulin receptor substrate (IRS)-1 and IRS-2 in the epididymal adipose tissue. On the other hand, in the liver, mRNA expression of InsR and IRS-1 did not change at any age, although that of the IRS-2 was reduced. Thus, in ddY-H mice, insulin resistance and glucose-stimulated hyper-secretion of insulin are induced at 9 weeks of age and are reciprocally affected, resulting in progression to a more severe state at an older age. Insulin resistance may be attributed, at least in part, to the decreases in the mRNA expressions and proteins of InsR, IRS-1 and IRS-2 in adipose tissue. Key words insulin resistance; age-related onset; insulin secretion; insulin receptor Type 2 diabetes mellitus is a complex disease characterized by hyperglycemia resulting from impaired pancreatic β-cell function and a decreased action of insulin on target tissues.