To determine whether improvement of insulin resistance decreases blood pressure as well as obesity, metformin (100mg/kg/d) or vehicle was administered for 20 weeks to 12-week-old male Otsuka Long-Evans Tokushima Fatty (OLETF) rats (n=10 each), a newly developed animal model of non-insulin-dependent diabetes mellitus (NIDDM) with mild obesity, hyperinsulinemia, and hypertriglyceridemia. Oral administration of metformin ameliorated glucose intolerance and attenuated the insulin response to glucose loading (2g/kg, i.p.), as evidenced by a decrease in the area under the curve for glucose and insulin at 24 weeks by 19% and 37%, respectively. At 21 weeks, systolic blood pressure was significantly lower in the metformin group than in controls (130±1.9 vs. 143±2.7mmHg, p<0.01), despite no difference in body weight. Subsequently, blood pressure tended to be slightly but insignificantly lower in the metformin group, and body weight was significantly lower in the metformin group (532±9.8 vs. 587±10.3g at 31 weeks, p<0.01). Metformin treatment also lowered the level of serum triglycerides (9.4±0.6 vs. 13.2±0.5mmol/l, p<0.01) and the plasma norepinephrine concentration (4, 222±373 vs. 7, 548±1, 058pg/ml, p<0.01). These results suggest that metformin-induced improvement of insulin resistance in obese rats with NIDDM may lower blood pressure, as well as decrease sympathetic activity and reduce body weight. (Hypertens Res 1996; 19: 37-41)