Previous studies have shown that the process of recovery after disuse muscle atrophy varies in different experimental models. We investigated the process of recovery of M, soleus atrophy after immobilization, with special reference to changes in structural and soluble proteins by means of electrophoresis and histochemical changes using myosin-ATPase staining. After rabbits had been subjected to 3 weeks of hindlimb immobilization, the changes were investigated immediately after the termination of immobilization, and at 1, 2, 3, 4 and 6 weeks of recovery. Just after the termination of immobilization, the wet weight of the M. soleus was 32.7±14.0% that of the opposite limb. The wet weight recovered rapidly and returned to the same level as that of the opposite side by 4 weeks. The amount of structural proteins decreased after immobilization, but did not return to the control level at 6 weeks after recovery. No new bands appeared in the electrophoretic patterns of the structural and soluble proteins at any of the stages of recovery. Furthermore, the special features of the bands of myosin light chains (MLCs-1, MLCs-2) also showed no change. ATPase staining showed that the area of type 2 fibers increased, and occupied 45.2±12.6% of the total area at 3 weeks after recovery, whereas that of muscle from controls occupied 17.3±5.7%. The area did not return too the control level by 6 weeks of recovery. The discrepancy between the histochemical changes and the changes in structural or soluble proteins during the recovery process appeared to be due to differences in the turnover ratio of each protein. It appears that the process of recovery of disuse muscle atrophy after immobilization is variable and compley, and differs according to the method of observation.