Metformin is a hypoglycemic drug effective in the treatment of non-insulin-dependent diabetes mellitus and increasingly used in Canada and Europe. Effects on intestinal glucose absorption, insulin secretion, and hepatic glucose production are insufficient to explain its hypoglycemic action, with most evidence suggesting that the major effect of the drug is on glucose utilization. In vivo and in vitro studies have demonstrated that metformin stimulates the insulininduced component of glucose uptake into skeletal muscle and adipocytes in both diabetic individuals and animal models. This increase is more significant in diabetic than in nondiabetic animals, suggesting an enhanced action of the drug in the hyperglycemic state. The increase in glucose uptake is also reflected in an increase in the insulin-dependent portion of glucose oxidation. Potential sites of action of metformin are the insulin receptor and the glucose transporters. Although metformin increases insulin binding in various cell types, this effect is not universal and does not correlate with stimulation of glucose utilization. In contrast, direct effects of the drug on the glucose-transport system have been demonstrated. Metformin elevates the uptake of nonmetabolizable analogues of glucose in both nondiabetic rat adipocytes and diabetic mouse muscle. In the latter, the stimulatory effect of the drug is additive to that of insulin. In human and rat muscle cells in culture, metformin increases glucose-analogue transport independently of and additive to insulin, suggesting an insulin-independent action. Most of these results suggest that the basis for the hypoglycemic effect of this biguanide is probably at the level of skeletal muscle by increasing glucose transport across the cell membrane.