BACKGROUND: Non-Insulin Dependent Diabetes Mellitus (NIDDM) is a characterized by insulin resistance and impairment of beta cell function. OLETF male rat usually developed NIDDM and obesity at 20 weeks old spontaneously. It is a metabolically characterized by insulin resistance in onset of early disease. However, body weight and insulin secretory function was gradually reduced during the diabetes developed. These characteristics of disease is similar to Korean type 2 diabetic patients. NIDDM patients in Korea are thought to be different from traditional NIDDM in western countries. They are non obese type and also has reduced insulin secretory function compared to western countries. These patients are not easily managed on diet and/or oral hypoglycemic agent. Reduced C-peptide and insulin concentrations in these patients are similar to patients with IDDM. In these patients, insulin therapy is effective to control glucose level. Therefore, we investigated the effect of insulin and oral hypoglycemic therapy to glucose control and severity of chronic complications in OLETF male rats of 6weeks (42 weeks old) and 14 weeks (50 weeks old) treated groups. MATERIAL AND METHODS: The OLETF male rats which are 36 weeks old is diagnosed to NIDDM. A total of 20 rats were stratified into the three groups: control group (n=3), OHA's group; rats treated by OHA's (n=3) and insulin group; rats treated with insulin (n=4). We evaluated anthropometry, fasting glucose and 75 gram OGATT, nerve conduction studies, sclerotic degree of kidney and thickness of carotid arteries at 42 and 50 weeks old. RESULTS: In the 42 weeks old groups (6 weeks treated group), there was a significant difference in weight gain in group 3 but no differences were observed in kidney tissue pathology and thickness of carotid arteries. In the 50 weeks old groups (14 weeks treated group), there were also no changes in the kidneys and arteries, but weight gain and peak amplitude in NCV was significantly higher in insulin - treated group. CONCLUSIONS: OLETF male rats as NIDDM animal mocel, with late stage diabetic complications show weight loss and decreased insulin secretory capacity. Insulin treated group shows improved blood glucose control. Also it showed improved severity of diabetic neuropathy.