Background Resting vascular tone is low in the normal pulmonary circulation, and experimental studies have suggested that this may be due to the continuous release of endothelium-derived nitric oxide (NO), a locally acting vasodilator. We have investigated whether NO contributes to the normal control of pulmonary vascular tone and resistance in children.Methods and Results We studied the hemodynamic effects of NG-monomethyl-l-arginine (L-NMMA), a specific inhibitor of NO synthesis, on the pulmonary circulation of six children 2 to 17 years old (mean, 9 years) with congenital heart disease but normal pulmonary blood flow, pressure, and resistance (all had isolated left heart obstructive lesions). The diameter of a segmental pulmonary artery and pulmonary blood flow velocity were measured by quantitative angiography and intra- arterial Doppler catheters. There was a consistent, dose-dependent fall in pulmonary blood flow velocity in response to three increasing doses of L-NMMA (compared with baseline, flow velocity fell to 75+-7%, 62+-8%, and 40+-10%, P<.01). Flow velocity returned to control values with subsequent infusion of l-arginine, the substrate for NO. Thereafter, acetylcholine, an endothelium-dependent dilator, produced an increase in flow velocity (56+-10% greater than baseline, P<.01). Arterial diameter was unchanged during L-NMMA and l-arginine infusions, indicating that the major effect of each agent is to alter vascular tone distal to the segmental pulmonary arteries.Conclusions The dilator action of endothelium-derived NO contributes to the maintenance of low resting pulmonary tone in normal children. Impairment of NO production may contribute to the elevated pulmonary vascular resistance that complicates some cases of congenital heart disease. (Circulation. 1994;89:2041-2044.)