Type II (non-insulin-dependent) diabetes mellitus is a polygenic disease characterised by insulin resistance in muscle, fat and liver, and the failure of pancreatic beta cells to adequately compensate for this resistance [1,2]. Glucose tolerance is reported to be impaired with advancing age [3,4]. Deterioration of glucose tolerance can be due to impaired insulin secretion or impaired insulin action or both. The relative contribution of insulin deficiency and insulin resistance to the pathogenesis of age-related glucose intolerance is still controversial [5] because of the difficulty in doing longitudinal studies in humans. Longitudinal analysis of glucose tolerance in animal models with different degrees of glucose intolerance is one of the best ways to address this question.The Nagoya-Shibata-Yasuda (NSY) mouse strain was established as an inbred animal model with spontaneous development of Type II diabetes, by selective breeding for glucose intolerance from out- Abstract Aims/hypothesis.The Nagoya-Shibata-Yasuda (NSY) mouse closely mimics human Type II (non-insulin-dependent) diabetes mellitus in that the onset is age-dependent, the animals are not severely obese, and both insulin resistance and impaired insulin response to glucose contribute to disease development. The aim of this study was to clarify the influence of age on the pathogenesis of diabetes and to analyse a candidate gene for Type II diabetes in this strain. Methods. Several phenotypic characteristics related to diabetes mellitus were monitored longitudinally in male NSY and control C3H/He mice. The nucleotide sequence of Glut4, a candidate gene for Nidd1nsy (a susceptibility gene for Type II diabetes) on Chromosome 11, encoding insulin-sensitive glucose transporter, was determined in NSY and C3H mice. Results. Glucose intolerance worsened with age, and fasting blood glucose and fasting plasma insulin concentration increased with age in NSY mice. Pancreatic insulin content increased until 24 weeks of age but then decreased at 48 weeks of age in NSY mice. The hypoglycaemic response to insulin was statistically significantly smaller in NSY than in C3H/He mice. The nucleotide sequence of GLUT4 cDNA was identical in NSY and C3H/He mice, but both were different from the sequence reported previously. Conclusion/interpretation. Insulin secretion and insulin resistance, as well as ageing possibly play an important part in the disease development in NSY mice. A decline of pancreatic insulin content in older age might cause the relative insulin deficiency in this strain. Nucleotide sequencing suggests that Glut4 is unlikely to be a candidate gene for Nidd1nsy. [Diabetologia (2000) 43: 932±938]