Objective To clarify the effects of simulated microgravity on the local regulating function of the pulmonary circulation. Method Rat tail-suspension(TS) was used as the animal model to simulate the effects of microgravity.The changes in responses of pulmonary artery to several vasoactive agents were determined by using in vitro vessel rings perfusion technique. Result As for the endothelium-intact vessel rings from TS group compared with those from the control(C) group,the contractile responses of pulmonary artery to potassium chloride (KCl,10～70 mmol/L) did not change obviously after 7 d tail-suspension,which were markedly diminished as to phenylephrine (PE,10-9～10-5mol/L).The vasodilatory responses to acetylcholine (ACh,10-9～10-5mol/L) were markedly enhanced,which did not change obviously as to captopril (Cap,10-5～3×10-3 mol/L).The vasoreactivity of endothelium-removed pulmonary artery rings to all vasoactive agents did not change after 7 d tail-suspension as compared with those from control rats. Conclusion The release of nitric oxide (NO) by rat pulmonary arterial endothelium was enhanced after 7 d tail-suspension.The vasodilatory function of pulmonary artery was enhanced.