Background: Previous studies in humans have shown that pacing-induced epicardial coronary artery dilation is nitric oxide dependent. The effects on this dilation of administration of L-arginine (the substrate for nitric oxide synthesis) is unknown.Methods: In 5 patients (4 male, 1 female; mean age 61 +/- 9 yrs) with coronary artery disease (CAD) and stable angina, atrial pacing (AP) 140 beats/min, was performed during the intracoronary infusion of normal saline (NS) and during infusion of 150 μmol/min L-arginine (LA). Coronary angiograms were recorded during the infusions, AP, and after intracoronary 250 mcg nitroglycerin (GTN). The diameter of 11 proximal and 12 distal segments was measured by quantitative angiography at baseline and after NS, AP + NS, AP + LA and GTN administration.Results: The mean (+/- SEM) diameters and % change from baseline were: Conclusions: The epicardial diseased coronary arteries dilate in response to atrial pacing. The addition of L-arginine augments nitric oxide dependent dilation induced by atrial pacing and therefore this may be beneficial in patients with atherosclerosis.