BACKGROUND Continuous treatment with nitroglycerin (GTN) causes tolerance and endothelial dysfunction, both of which may involve endothelial nitric oxide synthase (eNOS) dysfunction. eNOS dysfunction may be linked to depletion of tetrahydrobiopterin, and folic acid may be involved in the regeneration of this cofactor. It has been demonstrated that 10 mg⁄day folic acid supplementation prevents the development of GTN tolerance and GTN-induced endothelial dysfunction. However, the efficacy of daily lower-dose folic acid supplementation for preventing these phenomena has not been investigated. OBJECTIVE To determine the effect of 1 mg⁄day folic acid supplementation on responses to sustained GTN therapy. RESULTS On visit 1, 20 healthy male volunteers were randomly assigned to receive either oral folic acid (1 mg⁄day) or placebo for one week in a double- blind study. All subjects also received continuous transdermal GTN (0.6 mg⁄h). On visit 2, forearm blood flow was measured using venous occlusion strain-gauge plethysmography in response to incremental intra-arterial infusions of acetylcholine, N-monomethyl-L-arginine and GTN. Subjects in both groups displayed significantly decreased responses to acetylcholine and N-monomethyl-L-arginine infusions compared with a control group that received no treatment. Responses to GTN were also significantly diminished in both groups (P<0.05 for all). CONCLUSIONS The present data demonstrate that daily supplementation with 1 mg folic acid does not prevent the development of GTN-induced eNOS dysfunction or tolerance.