Abstract: The aim of this study was to investigate the effects of melatonin on glucose homeostasis in young male Zucker diabetic fatty (ZDF) rats, an experimental model of metabolic syndrome and type 2 diabetes mellitus (T2DM). ZDF rats (n = 30) and lean littermates (ZL) (n = 30) were used. At 6 wk of age, both lean and fatty animals were subdivided into three groups, each composed of ten rats: naive (N), vehicle treated (V), and melatonin treated (M) (10 mg/kg/day) for 6 wk. Vehicle and melatonin were added to the drinking water. ZDF rats developed DM (fasting hyperglycemia, 460 ± 39.8 mg/dL; HbA1c 8.3 ± 0.5%) with both insulin resistance (HOMA-IR 9.28 ± 0.9 versus 1.2 ± 0.1 in ZL) and decreased β-cell function (HOMA1-%B) by 75%, compared with ZL rats. Melatonin reduced fasting hyperglycemia by 18.6% (P < 0.05) and HbA1c by 11% (P < 0.05) in ZDF rats. Also, melatonin lowered insulinemia by 15.9% (P < 0.05) and HOMA-IR by 31% (P < 0.01) and increased HOMA1-%B by 14.4% (P < 0.05). In addition, melatonin decreased hyperleptinemia by 34% (P < 0.001) and raised hypoadiponectinemia by 40% (P < 0.001) in ZDF rats. Moreover, melatonin reduced serum free fatty acid levels by 13.5% (P < 0.05). These data demonstrate that oral melatonin administration ameliorates glucose homeostasis in young ZDF rats by improving both insulin action and β-cell function. These observations have implications on melatonin’s possible use as a new pharmacologic therapy for improving glucose homeostasis and of obesity-related T2DM, in young subjects.