Introduction: Type 2 diabetes (T2DM) is a chronic disease characterized by insulin resistance and/or abnormal insulin secretion, which prevalence is increasing in epidemic proportions worldwide. Furthermore, T2DM has been associated with a higher prevalence of cardiovascular disease, and recently it was associated with the risk of developing pulmonary arterial hypertension (PAH). The possibility that endothelial dysfunction causes PAH has been studied with contradictory results (1,2,3). Metformin is a widespread prescribed medicine indicated to treat insulin resistance and has been shown to improve endothelium function in rat diabetic models (4,5). Aims: Herein we have investigated the effect of acute and chronic metformin treatment in endothelial functionality in control and prediabetes animals. Methods: Experiments were performed in male Wistar rats aged 4 months. The prediabetes model used was the high-fat (HF) model, which is obtained by submitting the animals to a 60% lipid-rich diet during 3 weeks. This model was compared with age-matched controls that fed a standard diet. Endothelial function was evaluated by monitoring the relaxation effect of ACh over the contraction induced by prostaglandin F2α (PGF2α) or phenylephrine (PE) contraction. To evaluate the effect of chronic metformin on endothelial function, the drug was administered at a concentration of 200mg/kg/day in drinking water for the subsequent 3 weeks after starting the diet protocol, both in control and HF animals. Acute metformin effects were investigated by submitting the pulmonary artery to 3mM of metformin during 20 minutes. Results: HF diet did not alter endothelial function in the pulmonary artery, as the dose-response curves to ACh over PGF2α and PE contraction were similar between control and HF animals. Chronic metformin treatment showed a tendency to improve endothelial function in control and HF animals, as an intense relaxation in response to ACh over PGF2α was found, although without statistical significance. In contrast, while acute metformin administration did not affect the relaxation to ACh in control animals, it almost blocked completely the relaxation to ACh in the HF animals (ACh 30µM: as % of PGF contraction: 74.8±13.1 n=6 vs. 33.7±8.9, n=10 p<0.001; as % of PE contraction 93.4±4.5, n=6 vs. 52.0±8.1 n=10 p<0.001). Conclusions: Prediabetes does not cause significant endothelial damage to the pulmonary circulation. Additionally, chronic metformin treatment improved endothelial function over baseline. Interestingly, acute metformin treatment affected differently the endothelial function in controls and HF animals, suggesting that in HF animals, metformin interfered with the release of nitric oxide from the endothelium. As a whole this data suggest the existence of different mechanisms of action for the acute and chronic metformin effects on endothelial function.