Because metformin affects glucose and free fatty acid (FFA) metabolism in peripheral insulin target tissues, we investigated the effect of this drug in restoring a normal secretory pattern in rat pancreatic islets whose function has been impaired by chronic exposure to elevated FFA or glucose concentrations. We cultured rat pancreatic islets with or without FFA (2 mmol/l oleate/palmitate 2:1) or high glucose (16.7 mmol/l) concentrations in the presence or absence of metformin (0.25-12.5 microg/ml) and then measured insulin release, glucose utilization, glucose, and FFA oxidation. When compared with control islets, islets exposed to high FFA or glucose concentrations showed an increased basal and a decreased glucose-induced insulin release. In islets cultured for an additional 24 h with FFA or glucose in the presence of metformin (2.5 microg/ml), both basal and glucose-induced insulin secretions were restored. Both glucose utilization and glucose oxidation were altered in islets pre-exposed to high FFA or glucose concentrations. In particular, regarding control islets, glucose utilization was increased at 2.8 mmol/l glucose and decreased at 16.7 mmol/l glucose; glucose oxidation was similar to control islets at 2.8 mmol/l glucose but decreased at 16.7 mmol/l glucose. In contrast, oleate oxidation was increased in islets pre-exposed to FFA. All of these abnormalities were reversed in islets cultured for an additional 24 h with high FFA or glucose concentrations in the presence of metformin (2.5 microg/ml). In conclusion, our data show that metformin is able to restore the intracellular abnormalities of glucose and FFA metabolism and to restore a normal secretory pattern in rat pancreatic islets whose secretory function has been impaired by chronic exposure to elevated FFA or glucose levels. These data raise the possibility that, in diabetic patients, metformin (in addition to its peripheral effects) may have a direct beneficial effect on the beta-cell secretory function.