Our objective was to determine whether cyclooxygenase (COX) 2 dependent metabolites contribute to the altered pulmonary vascular responses that manifest in piglets with chronic hypoxia-induced pulmonary hypertension. Piglets were raised in either room air (control) or hypoxia for 3 days. The effect of the COX 2 selective inhibitor, NS398, on responses to arachidonic acid or acetylcholine (ACh) were measured in endothelium-intact and denuded pulmonary arteries (100-400 µm diameter). Pulmonary arterial production of the stable metabolites of thromboxane and prostacyclin were assessed in the presence and absence of NS398. Dilation to arachidonic acid was greater for intact control than for intact hypoxic arteries, was unchanged by NS398 in intact arteries of either group, and was augmented by NS398 in denuded hypoxic arteries. ACh responses, which were dilation in intact control arteries but constriction in intact and denuded hypoxic arteries, were diminished by NS398 treatment of all arteries. NS398 reduced prostacyclin production by control pulmonary arteries and reduced thromboxane production by hypoxic pulmonary arteries. COX 2 dependent contracting factors, such as thromboxane, contribute to aberrant pulmonary arterial responses in piglets exposed to 3 days of hypoxia.