Fourteen non-insulin-dependent diabetics (9 female, 5 male), aged 46 to 64 years, uncontrolled by diet (fasting plasma glucose greater than or equal to 8 mmol/l), were treated with metformin, 1-3 g daily, and followed prospectively at 1 week, then at 2-weekly intervals for 6 months. The fasting plasma glucose fell significantly (p less than 0.01) after 1 week of therapy and HbA1 showed a significant reduction, 13.4 +/- 2.5% (mean +/- SD) to 10.7 +/- 1.8% (normal range 5.1-9.3%) at 8 weeks (p less than 0.001). There was no significant change in body weight, plasma insulin, serum cholesterol or triglycerides, blood lactate, pyruvate, glycerol, alanine, 3-hydroxybutyrate or acetoacetate concentration. Systolic, diastolic, and mean arterial blood pressures, along with resting heart rate remained unchanged. This study shows that metformin effectively lowers plasma glucose without hyperinsulinism or significant rise in fasting blood lactate and other gluconeogenic precursors.