Abstract: Objective：To investigate the effect of simulated weightlessness on alveolocapillary permeability in healthy rats.Methods： Experiments were conducted on 40 Spragno-Dawley male rats，divided into 4groups：suspended for 7d，control of 7d and suspended for 21d，control of 21d.Tai-l suspension（—30°） was used as weightlessness simulation.The permeability index of fluorescein isothiocyanatedextran was used to characterize the alveolocapillary permeability and measured.The condition of vascular permeability factor in pulmonary tissues was examined by immunohistochemistry.Results：The permeability index of 7d tail-suspended rats increased significantly（P ＜0.01） and that of 21d also increased significantly（P＜0.05），but the permeability index of 21d tail-suspended rats increased less than that of 7d.The level of vascular permeability factor in pulmonary tissues of 7d tail-suspended rats increased significantly（P＜0.05） and that of 21d also increased significantly（P＜0.05），but the level of vascular permeability factor of 21d tail-suspended rats increased less than that of 7d.Conclusion：The alveolocapillary permeability increases during simulated weightlessness and can recover a bit as simulated weightlessness goes on.