Both endogenous and exogenous estrogen decrease pulmonary artery (PA) vasoconstriction. Whether these effects are mediated via estrogen receptor (ER)-alpha or ER-beta, and whether the contribution of ERs is stimulus-dependent, remains unknown. We hypothesized that administration of the selective ER-alpha agonist propylpyrazole triol (PPT) and/or the selective ER-beta agonist diarylpropiolnitrile (DPN) rapidly decreases PA vasoconstriction induced by pharmacologic and hypoxic stimuli via a nitric oxide (NO)-dependent mechanism. PA rings (n=3-10/group) from adult male Sprague-Dawley rats were suspended in physiologic organ baths. Force displacement was measured. Vasoconstrictor responses to phenylephrine (10-⁸M - 10-⁵M) and hypoxia (pO₂ 35-45 mmHg) were determined. Endothelium-dependent and -independent vasorelaxation were measured by generating dose-response curves to acetylcholine (10-⁸M - 10-⁴M) and sodium nitroprusside (10-⁹M - 10-⁵M). PPT or DPN (10-⁹M - 5x10-⁵M) were added to the organ bath in the presence and absence of the NO-synthase inhibitor L-NAME (10-⁴M). Selective ER-alpha activation (PPT, 5x10-⁵M) rapidly (<20 min) decreased phenylephrine-induced vasoconstriction. This effect, as well as PPT's effects on endothelium-dependent vasorelaxation, were neutralized by L-NAME. In contrast, selective ER-beta activation (DPN, 5x10-⁵M) rapidly decreased phase II of hypoxic vasoconstriction (HPV). L-NAME eliminated this phenomenon. Lower PPT or DPN concentrations were less effective. We conclude that both, ER-alpha and ER-beta decrease PA vasoconstriction. The immediate onset of effect suggests a nongenomic mechanism. The contribution of specific ERs appears to be stimulus-specific, with ER-alpha primarily modulating phenylephrine-induced vasoconstriction, and ER-beta inhibiting HPV. NO inhibition eliminates these effects, suggesting a central role for NO in mediating the pulmonary vascular effects of both ER-alpha and ER-beta.