Background and purpose: Disturbances in pulmonary vascular reactivity are important components of inflammatory lung disease. Haem oxygenase-1 (HO-1) is an important homeostatic enzyme upregulated in inflammation. Here we have investigated the potentially protective effect of HO-1 against cytokine-induced impairment in pulmonary artery relaxation. Experimental approach: Haem oxygenase-1 protein levels were assessed by immunofluorescence. HO activity was assessed by conversion of haemin to bilirubin. Rings of rat isolated pulmonary artery in organ baths were used to measure relaxant responses to the endothelium-dependent agent ACh and the endothelium-independent agent sodium nitroprusside (SNP). Production of nitric oxide (NO) and reactive oxygen species (ROS) was assessed by confocal fluorescence microscopy and fluorescent probes. Key results: Haem oxygenase-1 protein expression was strongly induced in pulmonary artery after 24-h incubation with either haemin (5 mu M) or curcumin (2 mu M), accompanied by a significant increase in HO activity. Incubation with tumour necrosis factor alpha (TNF alpha, 1 ng center dot mL-1, 2 h) significantly decreased relaxation of arterial rings to ACh, without affecting responses to SNP. Induction of HO-1 by curcumin or haemin protected against TNF alpha-induced hyporesponsiveness to ACh. The competitive HO inhibitor, tin protoporphyrin (20 mu M), abolished the protective effect of haemin. HO-1 induction prevented a TNF alpha-induced increase in NO generation without affecting the TNF alpha-induced increase in ROS generation. HO-1 induction prevented the TNF alpha-induced decrease in ACh-stimulated NO generation. Conclusions and implications: Induction of HO-1 protected against TNF alpha impairment of endothelium-dependent relaxation in pulmonary artery, by a mechanism involving a reduction in inducible NO synthase-derived NO production.