Mechanisms involved in the islet adaptation to insulin resistance were examined in mice of the C57BL/6J strain challenged with a high-fat (58%) diet for 8 weeks. Basal hyperglycemia commenced after 1 week, whereas hyperinsulinemia evolved after 8 weeks. Glucose elimination after an intravenous glucose challenge (1 g/kg) was significantly delayed after 1, 4, and 8 weeks on the high-fat diet compared with normal-diet-fed mice. This result was associated with unchanged insulin responses. However, glucose-stimulated insulin secretion from isolated islets was increased in a compensatory fashion at all glucose levels over a wide range (3.3-22 mmol/l) after 8 weeks on the high-fat diet, whereas no compensatory hypersecretion of insulin was evident after 1 or 4 weeks, except at 22 mmol/l glucose. Immunohistochemistry revealed that the islet architecture of insulin and glucagon cells remained intact in islets from mice fed a high-fat diet. However, the nuclear translocation of the homeobox transcription factor, pdx-1, and the plasma membrane translocation of GLUT2 were both impaired in high-fat-fed animals after 1 week. In contrast, the expression of the full-length leptin receptor (ObRb) was not affected by high-fat feeding. The study thus shows that 8 weeks are required for the development of a compensatory hypersecretion of insulin after high-fat feeding in mice, and even then the in vivo insulin secretion is insufficient to normalize impaired glucose tolerance. The early-onset islet dysfunction is accompanied by impaired beta-cell trafficking of two factors, pdx-1 and GLUT-2, which are involved in beta-cell proliferation and glucose recognition. The mechanisms compromising this beta-cell trafficking remain to be established.