Metformin, dimethylbiguanide, is an agent that has been widely used in the treatment of non-insulin-dependent diabetes mellitus. Its mechanism of action is not dependent on the stimulation of insulin secretion, unlike the sulfonylureas. It is clear that metformin reduces fasting endogenous glucose production rates, which are elevated in non-insulin-dependent diabetes mellitus. Some studies have proposed that metformin reduces endogenous glucose production primarily by reducing gluconeogenesis, whereas others have proposed that gluconeogenesis is not affected. The authors review the literature regarding the effects of metformin on hepatic carbohydrate metabolism.