Enhanced oxidative stress due to diabetes is accepted to lead to endothelial dysfunction, and this is known to play a key role in the pathogenesis of diabetic vascular diseases and complications. This study was designed to determine the possible protective effect of melatonin and/or insulin treatment on the functional and biochemical changes caused by hyperglycemia in aorta and corpus cavernosum of diabetic rats. Wistar albino male rats were rendered diabetic by injecting streptozotocin (60 mg/kg, intraperitoneally (i.p.)). Melatonin (10 mg/kg, i.p.) and/or insulin (6 U/kg, subcutaneously (s.c.)) were administered for 8 weeks. In the diabetic group, the contractile responses of aortic strips to phenylephrine were significantly impaired (EC50 5.5 x 10(-7) M in diabetic and EC50 1.47 x 10(-7) M in the control group, P<0.001). Treatment with melatonin (EC50 4.6 x 10(-7) M) or insulin+melatonin (EC50 1.68 x 10(-7) M, p<0.001) improved the contractile responses. Acetylcholine caused a dose-dependent relaxation response (EC50 1.58 x 10(-7) M) which was impaired in the diabetic group (EC50 26 x 10(-7) M, P<0.001). There was less impairment in melatonin-, insulin- and insulin+melatonin-treated groups (EC50 11.61 X 10(-7), 7.3 x 10(-7) and 1.41 X 10(-7) M, respectively, P<0.01). Contractile responses to phenylephrine were also impaired in the corpus cavemosurn strips (EC50 2.06 x 10-5 M in diabetic and 0.94x 1 0-5 M in the control group, P<0.001). In the melatonin- (EC50 1.59x 10(-5) M) and insulin+melatonin-treated (EC50 1.53 X 10(-5) M, P<0.5) groups contractile responses were improved. In the diabetic group, the relaxation responses of corpus cavemosum strips to acetylcholine were impaired (EC50 24.12 x 10(-5) M, P<0.001), and treatment with melatonin (EC50 0.68 x 10(-5) M), insulin (EC50 0.53 x 10(-5) M) or insulin+melatonin (0.98 x 10(-5) M, P<0.001) restored the responses to acetylcholine. In diabetic tissues, malondialdehyde levels were increased while glutathione levels were decreased, demonstrating oxidative damage. This was also prevented by treatment with melatonin or the melatonin and insulin combination. The diabetic state enhances the generation of free radicals, and both melatonin and insulin treatments reduced this oxidative stress; however, treatment with the combination was the most efficient in preventing diabetes-induced damage. Thus, our results suggested that giving diabetic patients adjuvant therapy with melatonin may have some benefit in controlling diabetic complications. (C) 2004 Elsevier B.V. All rights reserved.