The goal of this study was to determine, through a longitudinal follow-up, whether sex influences bone adaptation during simulated weightlessness. Twelve-week-old male and female Wistar rats were hindlimb unweighted for 2 wk, and the time course of bone alteration was monitored in vivo by means of densitometry and unbiased three-dimensional quantitative microcomputed tomography at 7 and 14 days. Compared with male rats, female rats had twice more cancellous bone volume at the proximal tibia at baseline, and this bone volume continued to increase, whereas in males it stabilized. Conversely, cortical area was greater in males than in females, and in both sexes cortical bone was still expanding. Hindlimb unloading resulted in larger reductions in males than in females in both cortical and cancellous compartments. In females, trabecular thickness and number decreased mildly, whereas in males trabecular number was dramatically reduced. In both sexes, the trabecular network became less connected and more rod-like shaped. Bone cellular activities evaluated by histomorphometry showed decreased bone formation rate in both sexes and increased resorption activity only in males. In conclusion, in female rats unloaded-related cancellous alterations reversed the growing process, whereas in males, which show lower growth process, it induced an accentuation of age-related cancellous bone changes for most of the parameters.