The morphological changes in the soleus muscle, immobilized by means of a plaster cast applied to the posterior limb of albino rats was studied. The animals were sacrificed 20 and 30 days after immobilization. The Authors agree with the hypothesis put forth by Lazarides regarding the cytoskeleton model of striated muscle fiber: morphological changes of the sarcomeres and myofilaments are brought on by translateral and longitudinal bridge damage.