Abstract: We studied the actions of endothelium derived relaxing factor （EDRF） on hypoxic pulmonary vasoconstriction（HPV） of isolated perfused pulmonary artery ring.The results showed that HPV was increased 80%（P＜ 0.01 ） by 10 -4 mol/L L NNA （an NO synthesis inhibitor ） or decreased 35% （P＜0.05） by 10 -2 mol/L L Arg （a precursor of NO）.Supplement of EDRF/NO,by addition of nitroglycerine to pulmonary artery rings,lowered HPV by 71% （P＜0.01） and this effect was partially blocked by methylene blue,guanylic acid cyclase inhibitor.Northern blot showed that hypoxia suppressed mRNA expression of nitric oxide synthase（NOS） in cultured endothelial cells from bovine pulmonary artery.The results suggest that decrease of NOS mRNA expression and,in turn,decrease of NO synthesis/release may be related to HPV pathogenesis.