Weightlessness results in negative calcium balance which can only reflect a redistribution of calcium in the body: the loss of calcium in the faeces and/or urine is constant, but an increase in urinary hydroxyproline indicating bone collagen destruction is not always detectable; moreover, a slowing down of collagen maturation may be suspected. Bone analysis by histomorphometry in animals and by indirect, non-invasive methods in man shows a decrease in bone mass. However, this bone tissue atrophy might only reflect excessive ageing of the bone during weightlessness, as suggested by slow bone formation and lack of variation in bone resorption. Since the experimental results obtained in men and animals during simulated weightlessness on earth are not strictly identical with those observed in space- flights, their validity may be questioned. Additional studies (notably histomorphometric studies) are therefore required for a better knowledge, as well as prevention, of the problems raised by human life in space.