Xu, D., Guo, S. and Liu, M. 2014. Effects of long-term simulated microgravity on tomato seedlings. Can. J. Plant Sci. 94: 273–280. Whether plants can adapt to a long-term microgravity environment is crucial to their reproduction in bioregenerative life-support systems in space. This research investigated the effects of simulated microgravity on Lycopersivon esculentum Mill. (cv. Dwarf Red-bell). Several indicators, namely germination ratio, percentage of cell membrane damage, malondialdehyde content (MDA), superoxide anion ([Formula: see text]) content, and mininucleolus, were observed 10, 20, 30, and 40 d after planting (DAP). Simulated microgravity [random positioning machine (RPM) treatment] barely had any effect on germination ratio, but it increased MDA, an index indicating membrane lipid peroxidation. Random positioning machine-treated samples had significantly higher [Formula: see text] content until 16 DAP, but these differences ceased after 21 DAP. Simulated microgravity damaged cell membranes, and the damage severity was positively related to the duration of the simulated microgravity treatment. Mininucleoli were more common in RPM-treated root tips than in the 1×g ones. In conclusion, simulated microgravity seriously disturbed tomato seedling growth by damaging cell membrane integrity, causing the accumulation of hazardous substances, and affecting the cell nucleus structure.