Studies have shown that tetracycline class antibiotics exhibit an ameliorating action with its antioxidant property on increased oxidative stress in tissues, including heart. Since endothelial vascular dysfunction in diabetes is associated with increased oxidative stress and prevented with antioxidants, herein, we aimed to test a hypothesis whether a low-dose doxycycline treatment of diabetic rats for 4 weeks can ameliorate endothelial vascular dysfunction of thoracic aortas. Results of the present study shows that both direct and alpha receptor-mediated contractile responses as well as endothelium-dependent and endothelium-independent vasodilatory responses were preserved with low-dose doxycycline treatment (30 mu mol/kg, daily; for 4 weeks) compared with untreated diabetic group. Furthermore, doxycycline treatment normalized increased lipid peroxidation and cellular glutathione level measured in plasma and prevented diabetes-induced impaired body weight gain without significant effect on high blood glucose level. Increased membrane protein level of caveolin-1, elevated ratio of PKC in particulate and cytosolic fraction, and increased protein level of cytosolic endothelin-1 in diabetic rats were also significantly prevented with doxycycline treatment. Moreover, diabetes-induced another type of oxidative stress markers in rats, matrix metalloproteinases, MMP-2, and MMP-9 were also normalized with doxycycline treatment in blood. Taken together, our data address that amelioration and/or prevention of vascular endothelial and contractile dysfunction by doxycycline is accompanied by a clear reduction in oxidative stress markers of diabetes, which provides evidence for doxycycline's potential antioxidant action as a therapeutic agent for amelioration and/or prevention of vascular disorders in diabetic subjects.