Loss of the modulatory role of the endothelium may be a critical initial factor in the development of diabetic vascular diseases. Exposure of human aortic endothelial cells (HAECs) to high glucose (30 or 44 mmol/l) for 7–10 days significantly increased the release of superoxide anion in response to the calcium ionophore A23187. Nitrate, a breakdown product of peroxynitrite (ONOO⁻), was substantially increased in parallel with a decline in cyclic guanosine monophosphate (GMP). Using immunochemical techniques and high-performance liquid chromatography, an increase in tyrosine nitration of prostacyclin (PGI₂) synthase (PGIS) associated with a decrease in its activity was found in cells exposed to high glucose. Both the increase in tyrosine nitration and the decrease in PGIS activity were lessened by decreasing either nitric oxide or superoxide anion, suggesting that ONOO⁻ was responsible. Furthermore, SQ29548, a thromboxane/prostaglandin (PG) H₂ (TP) receptor antagonist, significantly reduced the increased endothelial cell apoptosis and the expression of soluble intercellular adhesion molecule-1 that occurred in cells exposed to high glucose, without affecting the decrease in PGIS activity. Thus, exposure of HAECs to high glucose increases formation of ONOO⁻, which causes tyrosine nitration and inhibition of PGIS. The shunting of arachidonic acid to the PGI₂ precursor PGH₂ or other eicosanoids likely results in TP receptor stimulation. These observations can explain several abnormalities in diabetes, including 1) increased free radicals, 2) decreased bioactivity of NO, 3) PGI₂ deficiency, and 4) increased vasoconstriction, endothelial apoptosis, and inflammation via TP receptor stimulation.