Renal arteries (RAs) dilate in response to hypoxia, whereas the pulmonary arteries (PAs) constrict. In the PA, O₂ tension is detected by an unidentified redox sensor, which controls K⁺ channel function and thus smooth muscle cell (SMC) membrane potential and cytosolic calcium. Mitochondria are important regulators of cellular redox status and are candidate vascular O₂ sensors. Mitochondria-derived activated oxygen species (AOS), like H₂O₂, can diffuse to the cytoplasm and cause vasodilatation by activating sarcolemmal K⁺ channels. We hypothesize that mitochondrial diversity between vascular beds explains the opposing responses to hypoxia in PAs versus RAs. The effects of hypoxia and proximal electron transport chain (pETC) inhibitors (rotenone and antimycin A) were compared in rat isolated arteries, vascular SMCs, and perfused organs. Hypoxia and pETC inhibitors decrease production of AOS and outward K⁺ current and constrict PAs while increasing AOS production and outward K⁺ current and dilating RAs. At baseline, lung mitochondria have lower respiratory rates and higher rates of AOS and H₂O₂ production. Similarly, production of AOS and H₂O₂ is greater in PA versus RA rings. SMC mitochondrial membrane potential is more depolarized in PAs versus RAs. These differences relate in part to the lower expression of proximal ETC components and greater expression of mitochondrial manganese superoxide dismutase in PAs versus RAs. Differential regulation of a tonically produced, mitochondria-derived, vasodilating factor, possibly H₂O₂, can explain the opposing effects of hypoxia on the PAs versus RAs. We conclude that the PA and RA have different mitochondria.