By electron microscopy ultrastructures of myofibers of the soleus and gastrocnemius muscles of rats flown in zero-g and 1 g were examined. It was found that 4.5--9 hours postflight the soleus of weightless rats showed local destruction of the contractile and mitochondrial systems accompanied by metabolic changes in muscle fibers. Exposure to artificial gravity partially prevented changes in muscle fibers. Examinations of muscle fibers 25 days postflight demonstrated that the changes were reversible. No changes were seen in the mixed -- gastrocnemius -- muscle of both animal groups.