The effect of metformin treatment on various aspects of carbohydrate and lipoprotein metabolism has been defined in 12 patients with non-insulin-dependent diabetes mellitus (NIDDM). Patients were studied before and after approximately 4 mo of metformin therapy. Treatment was initiated with a single dose of 500 mg/day, increased at weekly intervals, and maintained at a final dose of 2.5 g/day (given at divided intervals) for the last 3 mo of the treatment program. Results demonstrated that both fasting and postprandial glucose concentrations were significantly lower after metformin administration, with the greatest change seen after meals. As a result, the total incremental plasma glucose response above basal measured from 0800 to 1600 after metformin was less than 25% of that seen initially. The improvement in ambient plasma glucose concentration in association with metformin occurred despite a modest but statistically significant decrease in circulating plasma insulin concentration. In addition, insulin-stimulated glucose uptake measured during hyperinsulinemic clamp studies was similar before and after metformin treatment. Furthermore, changes in insulin binding and insulin internalization by isolated monocytes did not correlate with the improvement in glycemic control. Thus, the ability of metformin to lower plasma glucose concentration in NIDDM does not appear to be secondary to an improvement in insulin action. Finally, metformin treatment was associated with a significant (P less than 0.01) decrease in plasma triglyceride concentration and an increase in plasma high-density lipoprotein cholesterol concentration. These results indicate that metformin treatment of patients with NIDDM led to an improvement in both glycemic control and lipoprotein metabolism.