Five pregnant growing rats were orbited for 5 days aboard the Soviet COSMOS 1514 biologic satellite. The bone effects of weightlessness were studied and compared to those of five pregnant rats kept in vivarium and five pregnant conditioned rats. Bone histomorphometric studies were performed to investigate the early effects of weightlessness in loaded (tibia-femur) and unloaded (thoracic and lumbar vertebrae) bones. A short exposure to weightlessness does not induce any change in bone mass and inner structure in either type of bone. In unloaded bones, the number of osteoclasts per square millimeter of the trabecular surface significantly increased when measured after histochemical demonstration of tartrate-resistant acid phosphatase. It is likely that a stimulation of bone resorption activity occurs in the trabeculae of unloaded bones during the early phase of a spaceflight. In tibia, osteoid seam thickness and total osteoclastic resorption surfaces at the endosteal level were not modified.