Both phosphodiesterase 5 (PDE5) inhibition and endothelin (ET) receptor blockade have been shown to induce pulmonary vasodilation. However, little is known about the effect of combined blockade of these two vasoconstrictor pathways. Since nitric oxide (NO) exerts its pulmonary vasodilator influence via production of cyclic guanosine monophosphate (cGMP) as well as through inhibition of ET, we hypothesised that interaction between the respective signalling pathways precludes an additive vasodilator effect. We tested this hypothesis in chronically instrumented swine exercising on a treadmill by comparing the vasodilator effect of the PDE5-inhibitor EMD360527, the ET_A/ET_B-antagonist tezosentan, and combined EMD360527 and tezosentan. In the systemic circulation, vasodilation by tezosentan and EMD360527 was additive, both at rest and during exercise, resulting in ??±?% drop in blood pressure. In the pulmonary circulation, both EMD360527 and tezosentan produced vasodilation. However, tezosentan produced no additional pulmonary vasodilation in the presence of EMD360527, either at rest or during exercise. Moreover, in isolated preconstricted porcine pulmonary small arteries (~300 μm) EMD360527 (1nM-10 μM) induced dose-dependent vasodilation, while tezosentan (1nM-10 μM) failed to elicit vasodilation irrespective of the presence of EMD360527. However, both PDE5-inhibition and 8Br-cGMP, but not cAMP, blunted pulmonary small artery contraction to ET and its precursor Big ET in vitro. In conclusion, in healthy swine, either at rest or during exercise, PDE5-inhibition and the associated increase in cGMP produce pulmonary vasodilation that is mediated in part through inhibition of the ET-pathway, thereby precluding an additive vasodilator effect of ET_A/ET_B receptor blockade in the presence of PDE5-inhibition.