The aim of this study was to determine if increased expression of heme oxygenase (HO) contributes to impairment of aortic contractile responses after hypoxia through effects on reactivity to endothelin-1 (ET-1). Thoracic aortas from normoxic rats and from rats exposed to hypoxia (10% O₂) for 16 or 48 hours were mounted in organ bath myographs for contractile studies, fixed in paraformaldeyde or frozen in liquid nitrogen for later protein extraction. In rings from normoxic rats, the HO inhibitor tin protoporphyrin IX (SnPP IX, 10 µmol/L) did not alter the response to phenylephrine or ET-1. In rings from rats exposed to hypoxia for 16 hours, maximum tension generated in response to these agonists was higher in endothelium-intact but not -denuded rings in the presence of SnPP IX. In rings from rats exposed to hypoxia for 48 hours SnPP IX increased contraction in endothelium-intact but not -denuded rings. In endothelium-intact aortic rings from rats exposed to hypoxia for 16 hours incubated with the endothelin A receptor-specific antagonist BQ123 (10^-7 M) , SnPP IX did not alter phenylephrineinduced contraction. Aortic endothelin-1 protein levels, measured by radioimmunoassay, were increased in rats exposed to hypoxia for 16 and 48 hours. Western blotting showed that HO-1 and HO-2 protein were increased after 16hrs of hypoxia and had returned to near control levels after 48 hours. The increase in HO-1 protein was detected in both endothelium-intact and denuded rings. Removal of the endothelium abolished the increase in HO-2 immunoreactivity. Immunohistochemistry localized the expression of HO-1 protein to the vascular smooth muscle whereas HO-2 was only detected in endothelium. HO-2 is expressed by aortic endothelial cells early during hypoxic exposure and impairs ET-1 mediated potentiation of contraction to -adrenoceptor stimulation.