Several congenital heart defects require surgery that acutely increase pulmonary blood flow (PBF). This can lead to dynamic alterations in postoperative pulmonary vascular resistance (PVR) and can contribute to morbidity and mortality. Thus, the objective of this study was to determine the role of nitric oxide (NO), endothelin (ET)-1, and their interactions, in the alterations of PVR following surgically induced increases in PBF. Twenty lambs underwent placement of an aortopulmonary vascular graft. Lambs were instrumented to measure vascular pressures and PBF, and studied for 4-hours. Before and following shunt opening, lambs received an infusion of saline (n=9), tezosentan, an ET_A and ET_B receptor antagonist (n=6), or N-nitro-L-arginine (LNA), a nitric oxide synthase (NOS) inhibitor (n=5). In control lambs, shunt opening increased PBF by 117.8% and decreased PVR by 40.7% (p<0.05) by 15 minutes, without further changes thereafter. Plasma ET-1 levels increased 17.6% (p<0.05), and total NOS activity decreased 61.1% (p<0.05) at 4 hours. ET-receptor blockade (tezosentan) prevented the plateau of PBF and PVR, such that PBF was increased and PVR was decreased as compared to controls at 3 and 4 hours (p<0.05). These changes were associated with an increase in total NOS activity (+61.4%, p<0.05) at 4 hours. NOS inhibition (LNA) following shunt placement prevented the sustained decrease in PVR seen in control lambs. In these lambs, PVR decreased by 15 minutes (p<0.05), but returned to baseline by 2 hours. Together these data suggest that surgically induced increases in PBF are limited by vasoconstriction, at least in part by an ET receptor mediated decrease in lung NOS activity. Thus, NO appears to be important in maintaining a reduction in PVR following acutely increased PBF.