The content of water-soluble proteins was investigated in the gray and white matter of the spinal cord, the spinal ganglia, and the sensomotor cortex of rats after a space flight lasting 18.5 days. The content of water-soluble proteins in the gray and white matter of the spinal cord and in the spinal ganglia of rats exposed to the action of weightlessness was significantly reduced after 4.5–9.5 h. In rats kept under conditions of artificial gravity during flight the content of water-soluble proteins was reduced in the white matter of the spinal cord. In animals kept previously under conditions of weightlessness 25 days after space flight a significant increase was found in the level of, water-soluble proteins in the gray matter of the spinal cord. In the gray matter of the sensomotor cortex of rats kept under conditions of weightlessness or of artificial gravity no changes were found (compared with controls kept in the ordinary animal house) in the content of water-soluble proteins whether 4.5–9.5 or 25 days after the satellite has landed.