Background : This study was aimed to observe the changes in body weight, water intake and hematological data during and after long-term head-down suspension (HOS) in rats. HOS rats induced by tail suspension has evolved as a useful model for the simulation of a microgravity or zero-gravity environment. Methods : Unanesthetized, unrestrained, male Sprague-Dawley rats, weighing 230-270 g, were subjected to either a horizontal position (control horizontal, CH rats) or a 45° head-down suspension (HOS rats) for 4 weeks. We determined the body weight and daily water intake in both CH and HOS rats. Hematological data including white and red blood cell counts, hemoglobin concentration and hematocrit were measured. The characteristics of red blood cell were also calculated from above hematological data in the both rats after 4 weeks of observation. Result: In CH rats, body weight was significantly increased (P<0.05) after 4 weeks of experiment, whereas the body weight of HOS rats was only showed a small increase but was significantly decreased (P<0.05) after 4 weeks of HOS compared with that of CH rats. Daily water intake was increased (P<0.05) after 2 week in HOS rats but the overall changes of the rest period were no difference between both CH and HOS rats. White blood cell counts did not show any changes in both rat groups while red blood cell counts showed a decreasing trend after 4 weeks HOS. After HOS, hemoglobin concentration, hematocrit and mean corpuscular hemoglobin were significantly decreased (P<0.05) compared with CH rats. Conclusion : These results represent that the long-term HOS could attenuate the increase in body weight of young rats and induce the decrease in hematological data, especially in the characteristics of red blood cell.