Recent evidence indicates that chronic hyperhomocysteinemia, which is found in from 9 to 15% of the general population, is an independent risk factor for the development of atherosclerosis. We sought to elucidate the mechanism by which exposure of the vascular wall to high levels of homocysteine initiates this inflammatory reaction. We examined the acute effect of homocysteine on endothelial dysfunction in isolated rat arteries and on microcirculatory leukocyte-endothelium interaction in vivo. Intravital microscopy of rat mesenteric venules was performed by superfusing the mesentery with increasing concentrations of homocysteine (1-5 mmol/l). There was a significant concentration- and time-dependent increase in leukocyte rolling, adherence, and extravasation compared with control rats superfused with Krebs-Henseleit solution (p < 0.01). Moreover, immunohistochemical staining demonstrated significantly increased P-selectin and intercellular adhesion molecule-1 (ICAM-1) expression on intestinal venules after homocysteine superfusion. In contrast, mesenteric superfusion with the nitric oxide donor 4-hydroxymethyl-furazan-3-carboxylic acid oxide (CAS1609, 1 micromol/l) significantly attenuated homocysteine-induced leukocyte rolling, adherence, and transmigration to control levels (p < 0.01). CAS1609 also attenuated both P-selectin and ICAM-1 expression on mesenteric venules and decreased CD18 expression on isolated leukocytes. Superior mesenteric arteries incubated with 5 mmol/l homocysteine developed significant (p < 0.01) endothelial dysfunction (i.e., impaired relaxation to endothelium-dependent dilators). Acute hyperhomocysteinemia induces endothelial dysfunction, characterized by a loss of endothelium-derived nitric oxide, leading to an inflammatory state. This state results in increased leukocyte rolling, adherence, and transmigration by upregulation of cell adhesion molecules. Our data suggest that hyperhomocysteinemia inhibits the important homeostatic role of nitric oxide in preventing endothelial dysfunction.