Nitric oxide (NO) regulates human vascular tone, with dysfunctional release contributing to vascular disease, effects that have been attributed to NO produced by endothelial NO synthase (eNOS)¹. However, we have recently shown that local infusion of the selective nNOS inhibitor S-methyl-L-thiocitrulline (SMTC) causes a dose-dependent reduction in plethysmographically-measured basal forearm blood flow at 10-fold lower doses than the non-selective NOS inhibitor N^Gmonomethyl-L-arginine (L-NMMA), without affecting eNOS-mediated acetylcholine-induced vasodilatation. These data suggest that basal forearm blood flow in humans is regulated by local nNOS-derived NO. Here, we studied the in vivo effects of nNOS on human forearm conduit arteries and the coronary circulation. The effects of brachial artery infusion of SMTC on radial artery diameter, flow and flow-mediated dilatation (FMD) were studied in healthy volunteers, using ultrasound². SMTC (0.2 µmol/min) reduced radial blood flow, calculated using the Doppler velocity time integral (VTI), by 35.9±8.1% (n=8; mean±SE; p<0.05 paired t-test), but did not affect radial artery diameter (p=ns) or FMD (5.9±0.6% before versus 6.6±0.6% after SMTC; p=ns). In the same subjects studied on a separate occasion, L-NMMA (2µmol/min) similarly reduced radial flow by 39.7±11.8% (p<0.01), and did not affect radial artery diameter (p=ns) but significantly inhibited FMD (6.5±0.2% before versus 1.0±0.3% after L-NMMA; p<0.0001). The effects of SMTC in the coronary circulation were studied following intracoronary infusion in patients with angiographically normal coronary arteries undergoing cardiac catheterisation. SMTC (0.625µmol/min) reduced basal coronary flow (Doppler flowire³) by 34.8±6.3% (n=8; p<0.01) but had no effect on vasodilatation induced by intracoronary substance P (20pmol/min). In contrast, L-NMMA (25µmol/min) reduced coronary flow by 22.3±6.1% and inhibited the flow response to substance P by 55±6.1% (n=7; p<0.01). SMTC caused a small but significant reduction in epicardial artery diameter (-2.8±0.9%; p=0.01), but did not affect conduit artery dilatation to substance P (6.3±1.4% before versus 6.6±1.8% after SMTC; p=ns). L-NMMA similarly reduced conduit artery diameter (-2.4±0.7%; p<0.05) but also reduced dilatation to substance P (6.0±1.4% before versus 3.3±0.9% after L-NMMA; p= 0.06). Blood flow and epicardial artery responses to 1mg bolus intracoronary isosorbide dinitrate were unaltered by SMTC or L-NMMA. These data suggest that local nNOS-derived NO regulates basal blood flow in both the human forearm and coronary vascular beds, whereas acetylcholine- or substance P-stimulated vasodilatation and FMD are eNOS-mediated. nNOS also appears to have a small effect on basal epicardial coronary artery tone.