It has been recently suggested that the small intestine (SI) has the capacity to contribute to endogenous glucose production (EGP), in addition to the liver and kidney. The aim of this work was: (1) to estimate the role of SI glucose fluxes in glucose homeostasis in insulin resistance states (induced by high-fat (HF) feeding); (2) to assess the effect of metformin, an anti-diabetic molecule, on these fluxes. Rats were fed for 6 weeks on a HF-diet, supplemented or not with metformin (HF-Met) at a daily dosage of 50 mg/kg during the last week. We combined arterio-venous glucose balance measurements and isotopic dilution techniques to separate basal intestinal glucose uptake (IGU) and release (IGR). Contrary to what was observed in control starch-fed rats, IGU was negligible in HF-fed rats: 0.6 +/- 2.4 micromol/kg min (mean +/- S.E.M., n = 9). It was restored to a level close to that of control rats in the HF-Met group: 13.0 +/- 6.7 micromol/kg min (mean +/- S.E.M., n = 9, p < 0.05 compared to the non-treated group). Similarly, IGR was close to zero in HF-fed rats (-3.8 +/- 2.6 micromol/kg min), but was significant in HF-Met rats (7.4 +/- 4.4 micromol/kg min, p < 0.05 compared to non-treated rats). These data strongly suggest that the impairment of glucose uptake in the SI might be a crucial feature of insulin resistance states and that a key beneficial effect of metformin in these situations might be to restore a normal glucose metabolism in this tissue.