Background —Loss of endothelial NO production after arterial injury may contribute to restenosis, characterized by neointima formation and elastic recoil. Adenovirus-mediated transfer of the gene encoding NO synthase (NOS) in balloon-injured arteries may restore NO production and inhibit neointima formation. Methods and Results —After balloon injury, rat carotid arteries were transduced with 3×10 ¹⁰ pfu/mL recombinant adenovirus carrying the human endothelial constitutive NOS cDNA (AdCMVceNOS, n=8) or no cDNA (AdRR5, n=8). ceNOS expression was confirmed by immunoblot analysis of vascular extracts and was localized by immunostaining in 30% of medial smooth muscle cells (SMCs) and in the adventitia of AdCMVceNOS-transduced arteries. Vascular cGMP levels were reduced from 3.9 pmol/g wet wt in uninjured arteries to 0.7 pmol cGMP/g after AdRR5 but were restored after ceNOS gene transfer (3.8 pmol cGMP/g wet wt, P <.05 versus AdRR5). Intima-to-media ratio 2 weeks after injury was significantly reduced (0.19±0.02 in AdCMVceNOS-infected versus 0.69±0.07 in AdRR5-infected arteries, P <.05). In vitro, BrdU incorporation of AdCMVceNOS-infected SMCs was reduced by 28% compared with AdRR5-infected SMCs. Transduced cells from injured carotid arteries subjected to FACS sorting showed a significantly lower BrdU labeling index in ceNOS-infected rats (29±6% versus 43±5% and 45±4% in control, injured, and AdRR5-infected rats, respectively, P <.05). Conclusions —AdCMVceNOS gene transfer to balloon-injured rat carotid arteries restores vascular NO productionand reduces neointima formation, at least in part because of an antiproliferative effect on medial SMCs. Adenovirus-mediated ceNOS gene transfer might reduce arterial restenosis after balloon angioplasty.