BACKGROUND Nitric oxide (NO), in addition to its potent vasorelaxant properties, may participate in growth regulation of cultured smooth muscle cells. It was recently demonstrated that in vivo endothelial injury induces the production of NO from L-arginine in the arterial wall. RESULTS We studied the effects of long-term administration of L-arginine, the precursor of NO, on neointimal thickening and on neoendothelium-dependent vasorelaxation 4 weeks after balloon denudation of normocholesterolemic rabbit iliac arteries. Rabbits were fed with either a standard diet or a diet supplemented with L-arginine (2.25%) in their drinking water 3 days before and during 4 weeks after balloon denudation. The effectiveness of L-arginine supplementation was confirmed by measurement of plasma arginine levels. L-Arginine had no effect on hemodynamic parameters. All animals were killed 4 weeks after balloon denudation, and a digital histomorphometric analysis of three serial nonconsecutive histological cross sections per iliac artery was performed. Intimal thickening was reduced (P < .05) from 0.43 +/- 0.08 (SE) mm2 in controls (n = 8) to 0.24 +/- 0.02 mm2 in treated animals (n = 8). Ten animals (n = 5 in each group) were used for in vitro vasoreactivity assessment 4 weeks after balloon denudation. Neoendothelium-dependent acetylcholine-induced relaxation (10(-8) mol/L to 3.10(-5) mol/L) in treated animals (Emax = -24.1 +/- 5.5%) was significantly greater than in controls (Emax = -8.9 +/- 2.2%). Endothelium-independent relaxation did not differ between groups (Emax = -58.1 +/- 6.5% in L-arginine-supplemented animals versus -52.9 +/- 6.8% in controls). CONCLUSIONS Our results demonstrate that L-arginine, a precursor of NO, reduces neointimal thickening after balloon denudation and improves neoendothelial-dependent acetylcholine-induced relaxation.