Background: Several drugs can cause prolonged QT interval, as well as prolonged QT dispersion (QTd) in electrocardiographic (EKG) recordings. QTd may be a potentially sensitive marker of increased risk of cardiac arrhythmias and sudden cardiac death. Metformin is an effective antihyperglycemic agent used in the treatment of diabetes. However, Studies have correlated dose-dependent effects of metformin on glycemia and cardiovascular risk markers. Objective: To evaluate the dose-response effects of metformin on QT and QTd of diabetic rats. Methods: Male Wistar rats were distributed in five groups: non-treated control (C), non-treated diabetics (D), diabetics treated with metmorfin at the doses of 3.5, 30 and 74 mu g/kg/bw (DM 3.5, DM 30 and DM 74). Diabetes was induced by an alloxan injection (40 mg/kg, IV). EKG was recorded (days 1, 15 and 30) using four elcctrodes inserted into the subcutaneous layer of the paws, Both RR and QT intervals were measured, and then corrected QT and QT dispersion values were calculated. Results: The DM 3.5 and DM 30 groups showed a significant reduction of glycomia (p < 0.05) when compared with the high dose (DM 74). Rats of the DM 74 group presented prolonged QTc, QTd and QTcd intervals, Whereas rats of the DM 3.5 and DM 30 groups presented less prolonged intervals. Conclusion: Metformin at high doses provided greater dispersion of the QT interval probably because of the increased ventricular repolarization inhomogeneity, whereas at low doses decreased QT intervals were observed in diabetic rats.904254260