The etiology of non-insulin-dependent diabetes mellitus (NIDDM) is not known. Hyperglycemia is due to increased hepatic glucose production (HGP), decreased glucose uptake, and impaired insulin secretion. It is unknown if these defects are coinherited or if one precedes and causes the others. The aim of this study was to determine the earliest defects in the evolution of the syndrome in the New Zealand obese (NZO) mouse, a polygenic model of NIDDM. NZO and control NZC mice were studied at 4–5 and 20 wk of age. Glucose turnover and glucose uptake in individual tissues were measured basally and during a hyperinsulinemic clamp. First-phase insulin secretion was measured after an intravenous glucose load. HGP was higher in the NZO mice both basally and during the clamp at bothages. At 4–5 wk of age, there was evidence of insulin insensitivity in brown adipose tissue, soleus, diaphragm, red quadriceps, and red gastrocnemius but not in heart, white quadriceps, and white gastrocnemius. In 20-wk-old mice, insulin responsiveness was decreased in white and brown adipose tissue and soleus muscle but not in heart, diaphragm, red and white quadriceps, and red and white gastrocnemius. First-phase insulin secretion (percentage rise above basal) 3 min after the glucose bolus was impaired in NZO miceat both ages. We conclude that hepatic glucose overproduction, brown adipose tissue and skeletal muscle insulin resistance, and impaired first-phase insulin secretion are all early abnormalities in the NZO mouse.