Objective: To study the effects of simulated microgravity induced by tail-suspension on histological structures and antioxidant system in testis of adult rats. Methods: Forty Spraque-Dawley adult male rats were randomly divided into four groups, two experimental groups and two synchronic control groups. Rats in the two experimental groups were tail-suspended for 14 d and 28 d respectively, then serum hormone, testicular morphology and biochemical changes were evaluated. Results:Compared with the synchronic controls, serum testosterone (T) levels and testicular superoxide dismutase (SOD) significantly decreased(P ＜ 0.05), while testicular malondialdehyde(MDA) significantly increased (P ＜ 0.05) in tail-suspended rats. The changes were more prominent in the 28 d tail-suspended rats. In addition, hematoxylin and eosin (HE) staining showed that seminiferous tubules atrophied, spermatogenic cells decreased, and seminiferous epithelial cells disintegrated and shed, abnormal multinucleated giant cells appeared and the lumina were azoospermic in tail-suspended rats. Conclusion: Simulated microgravity induced by tailsuspension has a harmful effect on male reproduction of adult rats by decreasing the ability of testicular antioxidant defense.