By light and electron microscopy long tubular bones of white Wistar rats were examined after a 22-day space flight and ground-based synhcroneous experiment. Approximately half of the flight rats showed loosened spongiose of metaphyses which was normally combined with a reduced area of the primary spongiose near the cartilaginous growth plate. This was suggestive of inhibited growth of bones in-flight. The light and electron microscopic examination of bones of flight rats revealed wide osteocyte cavities (lacunae) that could be associated with perilacunar osteolysis. The study of synchroneous animals demonstrated a less expressed spongiose of metaphyses found only in one third of the rats which was not usually combined with a reduced area of the primary spongiose near the cartilaginous growth plate. The histological examination of bones of flight rats on the 27th postflight day indicated that that period of time was insufficient for the normalization of all the changes.