The current study examined the responsiveness of blood vessels from diabetic rats to K+ channel openers and explored whether ROS might be involved in any changes. Responses were measured in aortic rings isolated from four weeks streptozotocin (65 mg/kg)-induced diabetic rats. Relaxation to levcromakalim (ATP-sensitive potassium channel KATP opener, 10(-9)-10(-5) mol/l) and (+/-)-naringenin (large conductance calcium-activated channel BKCa opener, 10(-8)-10(-3) mol/l) were recorded in phenylephrine (1 µmol/l) pre-contracted segments in the absence and presence of superoxide dismutase (SOD, 100 µmol/l) and apocynin (an antioxidant and inhibitor of NADPH oxidase, 100 µmol/l). Contractions to phenylephrine (10(-9)-10(-5) mol/l) and relaxation to acetylcholine (ACh, 10(-9)-10(-5) mol/l) were also recorded. Relaxation curves for levcromakalim, naringenin and ACh for the diabetic group were shifted to the right (p