['To investigate the dysfunction of vascular smooth muscle in streptozotocin-induced diabetic rats.', "Rings without endothelium of femoral arteries were suspended in organ chambers for isometric tension recording. The production of oxygen-derived free radicals was measured with 2',7'-dichlorodihydrofluorescein diacetate using confocal microscopy. The protein expressions were measured by western blotting.", 'The concentration-response curves to U46619 and phenylephrine, but not that to KCl, were shifted to the left, suggesting a hypersensitivity of cell membrane receptors in diabetes. Exogenous oxygen-derived free radicals induced greater vasoconstrictions in the femoral artery from diabetic rats. Chronic treatment with apocynin (inhibitor of NADPH oxidase) and acute exposure to MnTMPyP (SOD/catalase mimetic) normalized the response. The catalase activity and the total glutathione level were reduced in arteries from streptozotocin-treated rats, confirming a redox abnormality. The basal oxidative state was higher in arteries from streptozotocin-treated rats and reduced in arteries from apocynin- and streptozotocin-treated rats, suggesting that the functional changes in diabetes are due to a chronic increase in oxidative stress. In the arteries of streptozotocin-treated rats, inhibitors of COX-1 and/or COX-2 prevented the hypersensitivity and reduced the increase in oxidative stress caused by phenylephrine and U46619, suggesting that both isoforms contribute to the smooth muscle dysfunction. The expression of proteins for COX-1 and COX-2 was increased in arteries of streptozotocin-treated rats and reduced in preparations of apocynin- and streptozotocin-treated rats.'] Do oxidative stress and COX cause hyper-responsiveness in vascular smooth muscle of the femoral artery from diabetic rats?