Abstract: AIM: To investigate the effects of 3-week simulated microgravity and intermittent artificial gravity on apoptosis of thoracic aortic smooth muscle cells (TASMCs) in rats. METHODS: Twenty seven Sprague Dawley rats were randomly divided into three groups with nine rats in each group: control group (CON), tail-suspended group (SUS) and standing group (STD). SUS rats were used to simulate the effects of microgravity and homochronous countermeasure (STD, daily 1 h of -Gx gravitation) was used to simulate the effects of intermittent artificial gravity (IAG). Apoptosis of TASMCs was assessed by TUNEL staining, and protein expressions of Bad, FasL and Caspase-3 in thoracic aorta tissues were observed by Western blot. RESULTS: Compared with those in control (CON) group, TUNEL-positive TASMCs in STD increased significantly (P<0.01), and TUNEL positive TASMCs in SUS rats decreased significantly (P<0.01) compared with those in both CON group and SUS group. Compared with that in CON group and STD group, protein expression of Bad in the SUS group significantly decreased (P<0.05). Expression of Bad displayed an increasing tendency in STD group compared with that in the CON group, but without a significant difference. Compared with those in the CON group, protein expressions of either FasL or Caspase-3 in the SUS group decreased significantly (P<0.05), respectively, but expressions of both FasL and Caspase-3 in STD group increased significantly (P<0.01), respectively, compared with those in either the CON or SUS group. CONCLUSION: Decrease of apoptosis of TASMCs can be induced by simulated microgravity, whereas intermittent artificial gravity may cause the increase of apoptosis of TASMCs. These findings further suggest that apoptosis of vascular smooth muscle cells may play an important role in the adaptive remodeling of vessels induced by microgravity.