Tibia and lumbar vertebrae of rats sacrificed 2 days after their 13-day space flight on Cosmos-1887 were examined histomorphometrically. It was found that the exposure led to osteoporosis of the primary and secondary spongiosa of tibia proximal metaphyses and early signs of bone reduction in the lumbar vertebrae spongiosa. The major cause of osteoporosis was inhibition of bone neoformation which was indicated by a lower number and activity of osteoblasts. A certain contribution was also made by enhanced bone resorption which was proven by a greater number of osteoclasts per bone volume and area unit in the tibia primary spongiosa. The fact that the rats were exposed to 1 G for 48 hours after landing resulted in readaptation developments, which were pointed to by a decrease of acid phosphatase in osteoclasts.