Background The balance between vasodilation and vasoconstriction plays a maior role ln maintaining vascular homeostasis.However,the underlying mechanisms are unclear.More and more evidence suggested that there was an interaction in the regulation of vasorelaxation between nitric oxide(NO)and hydrogen sulfide(H2S).We explored the interaction between and effects of NO and H2S on the relaxation of pulmonaw arteries in rats.Methods Seven male Sprague-Dawley rats were anaesthetized with chloral hydrate and the pulmonary arteries of each rat separated for the study of vascular activities.The vasorelaxing activities of pulmonary artery rings in response to different doses of a NO donor,sodium nitroprusside(SNP),or a H2S donor,sodium hydrogensulfide(NaHS),were measured in vitro.When pulmonary artery rings were treated with a cystathionine-y-lyase inhibitor,DL-propargylglycine,in the presence of SNP or a nitric oxide synthase inhibitor, Nω-nitro-L-arginine methyl ester,in the presence of NaHS,the changes in relaxing activities were analyzed.Results The relaxation of pulmonary artery rings was in a dose dependent manner in response to either SNP or NaHS.The relaxation rates of pulmonaw artery rings increased from(30.90±4.62)%10(60.50±8.08)%when the concentration of SNP increased from 1 μmol/L to 3 μmol/L and from(26.13±4.12)%to(53.09±14.01)%when the concentration of NaHS increased from 25 μmol/L to 100 μmol/L.However,when appropriate inhibitor was added.the relaxation responses to SNP and NaHS decreased.Conclusions The results suggested that similarly to NO,H2S acted as a vasorelaxant either independently of,or synerqistically with NO in the regulation of vasorelaxation.The interaction between NO and H2S played an important role in regulating relaxing activities of pulmonary arteries.