Hypoxic pulmonary vasoconstriction (HVC), an intrinsic and assumed ubiquitous response of mammalian pulmonary blood vessels, matches regional ventilation to perfusion via an unknown oxygen sensing mechanism. Global pulmonary hypoxia experienced by individuals suffering from COPD or numerous hypoventilation syndromes including sleep apnea, often produces maladaptive pulmonary hypertension (PH) but PH is not observed in diving mammals where profound hypoxia is routine. Here we examined the response of cow and sea lion pulmonary arteries (PA) to hypoxia and observed the expected HVC in the former and a unique hypoxic vasodilation (HVD) in resistance vessels from the latter. We then used this disparate response to examine the O₂ sensing mechanism. In both animals, exogenous hydrogen sulfide (H₂S) mimicked the vasoactive effects of hypoxia in isolated PA. H₂S-synthesizing enzymes, cystathionine -synthase, cystathionine -lyase and 3-mercaptopyruvate sulfur transferase, were identified in lung tissue from both animals using one-dimensional western blot analysis and immunohistochemistry. The relationship between H₂S production/consumption and O₂ was examined in real-time, using amperometric H₂S and O₂ sensors. H₂S was produced by both sea lion and cow lung homogenate in the absence of O₂ but it was rapidly consumed when O₂ was present. Furthermore, consumption of exogenous H₂S by cow lung homogenate, pulmonary arterial smooth muscle cells and heart mitochondria was O₂-dependent and exhibited maximal sensitivity at physiologically relevant Po₂s. These studies show that HVC is not an intrinsic property of PA and provide further evidence for O₂-dependent H₂S metabolism in O₂ sensing.