Our knowledge of the adaptation of human bone microgravity remains poor despite long-term Russian spaceflights and the recent use of accurate techniques for bone mass measurements. The extent of bone deficits in the adaptation of the whole skeleton is not clear. At the tissue level, bone resorption and formation activities have been studied only in bones from rats after spaceflights lasting a few days to 3 weeks. In these animals, architectural features consistent with osteoporosis have been found in the proximal tibia. In pregnant animals the osteoclast population is increased at other skeletal sites. In areas of weight-bearing bones that are not protected by muscular insertions, bone resorption is not markedly altered after 7 days of spaceflight and bone formation is reduced. In areas of weight-bearing bones with muscular insertions and in non-weight-bearing bones, similar changes in bone cell activity are delayed. The severity of the response seems to vary with the location of the bone in the skeleton and its initial level of bone turnover. After 12.5 days the acute bone changes are less and no additional changes are observed after 21 days in space. We conclude that generalized bone deficits do not appear to be a consequence of microgravity but occur in localized areas according to the level of modeling and remodeling and of the support function of each bone at 1 g.