Recent studies show that pulmonary vasodilator responses to nitrite are enhanced by hypoxia. However, the mechanism by which nitrite is converted to vasoactive nitric oxide (NO) is uncertain. In the present study, iv injections of sodium nitrite decreased pulmonary and systemic arterial pressures and increased cardiac output. The decreases in pulmonary arterial pressure were enhanced when tone in the pulmonary vascular bed was increased with U46619. Under elevated tone conditions, decreases in pulmonary and systemic arterial pressures in response to nitrite were attenuated by allopurinol in a dose that did not alter responses to the NO donors, sodium nitroprusside and DEA/NO, suggesting that xanthine oxidoreductase (XOR) is the major enzyme reducing nitrite to NO. Ventilation with a 10% O₂ gas mixture increased pulmonary arterial pressure and the response to hypoxia was enhanced by L-NAME and not altered by allopurinol. This suggests that NO formed by the endothelium and not from the reduction of plasma nitrite modulates the hypoxic pulmonary vasoconstrictor response. Although iv injections of sodium nitrite reversed pulmonary hypertension responses to U46619, hypoxia and L-NAME, the pulmonary vasodilator response to nitrite was not altered by ventilation with 10% O₂ when baseline pulmonary arterial pressure was increased to similar values in animals breathing room air or the hypoxic gas. These data provide evidence that XOR is the major enzyme reducing nitrite to vasoactive NO and that this mechanism is not modified by hypoxia.