A progressive loss of bone mass is consistently reported in humans exposed to weightlessness during space flight. The mechanism of this metabolic alteration is unknown but may be due in part to the effects of disuse osteoporosis along with the influence of elevated corticosteroid levels. With a goal toward mankind's permanent presence in space, effective countermeasures against bone demineralization will be required. Synthetic anabolic steroids have been shown to prevent bone loss in ground-based immobilization studies and in excess corticosteroid states. These agents may offer a means of slowing or preventing bone loss thus facilitating long term space habitation.