Our previous study indicated that a single i.p. injection of 100 mg/kg streptozotocin (STZ) is able to induce slowly progressive diabetes mellitus in adult ICR mice. In the present study, the non-fasting serum insulin levels of the mice administered 100 mg/kg STZ were normal throughout the 24-week-observation after STZ injection. In the STZ-administered mice, the area of islets and the number of insulin-immunoreactive cells (beta-cells) were normal at 1 week and then continued to decrease gradually as the time went on. In contrast, there was a relative increase in the number of glucagon-immunoreactive cells (alpha-cells) in these mice. In addition, in the STZ-administered mice, the degree of glucose tolerance continued to reduce from 2 weeks till 12 weeks when the experiment terminated. The rise in serum insulin levels stimulated by glucose in the STZ-administered mice began to subside from about 2 weeks and had completely ceased by 12 weeks. These results indicate that 100 mg/kg STZ-induced diabetic mouse model is non-insulin-dependent diabetes, which is characterized by impaired insulin response to glucose stimulation.