We examined the involvement of the oxidative stress in high glucose-induced suppression of human aortic endothelial cell proliferation. Chronic glucose treatment for 72 h concentrationdependently (5.6-22.2 mol/l) inhibited human coronary endothelial cell proliferation. Temocaprilat, an angiotensin-converting enzyme inhibitor, at 10 nmol/l to 1 μmol/l inhibited high glucose (22.2 mmol/l)-mediated suppression of human aortic endothelial cell proliferation. Temocaprilat at 1 μmol/l inhibited high glucoseinduced membrane-bound protein kinase C activity in human aortic endothelial cells. The protein kinase C inhibitors calphostin C 100 nmol/l or chelerythrine 1 μmol/l inhibited high glucose-mediated suppression of human aortic endothelial cell proliferation. Chronic high glucose treatment for 72 h increased intracellular oxidative stress, directly measured by flow cytometry using carboxydichlorofluorescein diacetate bis-acetoxymethyl ester, and this increase was significantly suppressed by temocaprilat 10 nmol/l to 1 μmol/l. Bradykinin B2 receptor antagonist icatibant 100 nmol/l significantly reduced the action of temocaprilat; whereas bradykinin B1 receptor antagonist des-Arg9-Leu8-bradykinin 100 nmol/l had no effect. These findings suggest that high glucose inhibits human aortic endothelial cell proliferation and that the angiotensin-converting enzyme inhibitor temocaprilat inhibits high glucose-mediated suppression of human aortic endothelial cell proliferation, possibly through suppression of protein kinase C, bradykinin B2 receptors and oxidative stress.