Oxidative stress impairs endothelium-dependent dilation (EDD) with aging in healthy sedentary adults. Increased cytochrome P450 2C9 (CYP 450 2C9) signaling can contribute to oxidative stress-mediated suppression of EDD, but its role in aging is unknown. We hypothesized that inhibition of CYP 450 2C9 signaling with Sulfaphenazole would improve EDD in older, but not young, healthy sedentary adults. At baseline, increases in forearm blood flow (FBF; venous occlusion plethysmography) in response to brachial artery infusions of acetylcholine (ACh; 1, 2, 4, and 8 µg 100mL forearm volume min-1), an endothelium-dependent dilator, were smaller in older (n=14, 63±1 yr; mean±SE) vs. young (n=11; 23±2 yr) adults (P<0.05), with a reduction in peak FBF of 32% (11.8±1.7 vs.17.3±2.3 mL 100mL tissue-1 min-1). Infusion of Sulfaphenazole at doses that block CYP 450 2C9 signaling in humans did not affect the FBF responses to ACh in either the older (peak FBF: 13.0±4.3 mL tissue-1 min-1, P=0.41) or the young (peak FBF: 17.1±1.9 mL 100mL tissue-1 min-1, P=0.55) group. Co-administration of the nitric oxide inhibitor L-NMMA with Sulfaphenazole decreased the FBF response to ACh in both young and older subjects (P<0.05); the effect was smaller in the older subjects, but group differences in EDD remained (P<0.05). Endothelium-independent dilation assessed with sodium nitroprusside was not different in the young and older subjects. These results provide the first support for the concept that increased CYP 450 2C9 signaling does not contribute to impairments in EDD with aging in healthy adults.