Abstract-We hypothesized that in hyperhomocysteinemia (HHcy), flow-induced arteriolar constriction is due to an enhanced generation of reactive oxygen and/or nitrogen species, causing an impairment of nitric oxide (NO) and prostaglandin mediation of the response. Changes in diameter of isolated, pressurized (at 80 mm Hg) gracilis muscle arterioles (diameter Ϸ170 m) from control and methionine diet-induced HHcy rats were measured by videomicroscopy. Key Words: homocysteine Ⅲ arterioles Ⅲ flow-induced constriction Ⅲ thromboxane A 2 Ⅲ reactive oxygen species Ⅲ xanthine oxidase I ncreased plasma concentration of the methionine metabolite homocysteine 1 is an independent risk factor for cardiovascular diseases, such as stroke, myocardial infarction, peripheral vascular disease, and atherosclerosis. 1,2 One of the mechanisms by which hyperhomocysteinemia (HHcy) elicits its adverse vascular effects is by altering endothelial function. Recent studies by other laboratories and ours showed that even mild HHcy (15 to 30 mol/L) is associated with significant impairment of endothelium-dependent acetylcholine-, histamine-, or bradykinin-induced vasoactive responses in large arteries and aorta of monkeys 3,4 and mice, 5-7 as well as in rat skeletal muscle arterioles. 8,9 Furthermore, we demonstrated that increases in intraluminal flow-related wall shear stress, the primary stimulus for the endothelial synthesis/release of nitric oxide (NO) and prostaglandins (PGs) in vivo, which maintain a dilated state of arterioles, elicit constrictions in arterioles of HHcy rats because of the simultaneous lack of NO mediation and enhanced thromboxane A 2 (TxA 2 ) release. 10 These findings have important clinical applications, because in humans, a similar degree of mild HHcy, either basal or transient after a methionine load, is associated with an impaired dilation or even constriction of arteries in response to a release of forearm occlusion. [11][12][13][14][15][16] The underlying mechanisms responsible for flow-induced endothelium-dependent constriction in HHcy, however, remained unknown.Flow-dependent skeletal muscle, arteriolar dilation is mediated by NO and dilator PGs, the synthesis/release of which can be substantially affected by reactive oxygen species (ROS). 17 Studies on endothelial cells in culture suggest that high levels of homocysteine may promote the generation of ROS. 18 -22 Previous studies demonstrated that ROS [in particular, superoxide produced by endothelial NAD(P)H oxidase and/or xanthine oxidase] may impair arteriolar functions in pathophysiological conditions, such as hypertension, 23 hypercholesterolemia, 24 and diabetes. 25 A role for oxidative stress in HHcy-induced endothelial dysfunction in humans is also supported by recent studies showing that oral administration of antioxidants (eg, vitamin C, vitamin E) prevented oral methionine load-induced impairment of dilation of conduit arteries to acetylcholine 26 and after release of forearm occlusion. 27,28