We investigated the effect of dietary supplementation of L-arginine (L-Arg), the precursor of endothelial nitric oxide (NO), on endothelium-dependent and endothelium-independent vascular responses, as well as vascular structure, in the abdominal aorta of hypercholesterolemic rabbits. Rabbits were fed (a) normal rabbit chow, (b) 1% cholesterol diet, or (c) 1% cholesterol diet supplemented with 2.25% L-Arg HCl in drinking water. After 10 weeks, the abdominal aorta was harvested for study of vascular reactivity and histomorphometry. L-Arg did not affect serum cholesterol levels. Histomorphometric analysis demonstrated an eightfold reduction in intimal thickening in the abdominal aorta of the arginine-supplemented hypercholesterolemic rabbits. By contrast, the effects on vascular reactivity were subtle. Contraction to norepinephrine (NE) was not altered by hypercholesterolemia or L-Arg. Contraction to acetylcholine (ACh) was increased in hypercholesterolemic animals; this was normalized by dietary arginine supplementation. Relaxation to nitroglycerin (NTG) was not altered by hypercholesterolemia but was attenuated in the arginine-supplemented rabbits. Endothelium-dependent relaxation to ACh was impaired in both hypercholesterolemic groups. Dietary L-Arg has a dramatic antiatherogenic effect in hypercholesterolemic rabbits. This effect is associated with rather slight changes in vascular reactivity that are suggestive of a slight increase in NO elaboration by the endothelium. The discordance between the effects of dietary arginine on vascular structure and reactivity suggests that the antiatherogenic effects of the NO precursor may not be mediated entirely by its effect on the endothelium.