Endothelial dysfunction has been proposed as an early manifestation of atherosclerosis. The risk for atherosclerosis is increased in patients with diabetes mellitus, but the mechanism of the increased risk in these patients remains to be elucidated. Emerging evidence suggests that postprandial hyperglycaemia and hyperlipidemia are important risk factors in the development of atherosclerosis in patients with diabetes. Using a high-resolution ultrasound technique, we evaluated the acute effects of oral glucose loading on endothelium-dependent flow-mediated dilation (EFMD) and endothelium-independent flow-mediated dilation (EIFMD) of the brachial artery in 11 men (mean age: 59 +/- 5 years) with type 2 diabetes without chronic complications of diabetes. During these examinations, changes in the level of superoxide anion formation in the neutrophils were also measured. In addition, to investigate the relationship between acute hypertriglyceridemia and EFMD, we assessed the effects of high- and low-fat meals on EFMD of the brachial artery in 12 healthy volunteers. EFMD was diminished after glucose loading (13.2% +/- 6.4%, 7.3% +/- 3.3%*, 12.8% +/- 5.6%, in fasting and at 1 and 2 hours, respectively; *P<0.001 vs fasting). Superoxide anion formation by neutrophils (expressed as 10(-7) nmol/10(6) cells/30 min) was increased after glucose loading (4.7 +/- 2.8 and 6.2 +/- 2.2, in fasting and at one hour, respectively; P<0.05). EIFMD and triglyceride concentrations were not significantly affected by glucose loading. EFMD was also decreased by high-fat feeding (13.1% +/- 4.3%, 7.7% +/- 3.7%*, 7.3% +/- 2.2%*, basal, 2 hours, and 4 hours, respectively; *P<0.01 vs basal). These decreases were reversed by vitamin E treatment. These results show that acute hyperglycaemia induced by 75 gm oral glucose intake and acute hypertriglyceridemia induced by high-fat feeding are implicated in endothelial dysfunction. In addition, these results suggest that chronic and repeated hyperglycaemia and hypertriglyceridemia may play important roles in the development and progression of vascular complications in diabetes, probably through increased oxidative stress.