The purpose of this study was to determine the amount of soleus muscle atrophy in rats due to 28 days of decreased activity induced by hindlimb suspension, to observe the restoration of mass and relative muscle weight of the atrophied soleus muscle at day 28 of recovery to control value, and to compare the effect of run training on the mass and relative muscle weight of the atrophied soleus muscle at day 28 of recovery with that of sedentary rats. Adult female Wistar rats were maintained for 28 days with hindlimb suspension. Rats were then assigned randomly to a cage sedentary or running group. Soleus muscle mass and relative muscle weight following hindlimb suspension were compared with a control value. The soleus muscle mass and relative muscle weight of the running and cage sedentary groups following hindlimb suspension were compared with those of a control group. Soleus muscle mass and relative muscle weight of the run training group were compared with those of cage sedentary group. The results obtained were as follows : 1. Soleus muscle mass and relative muscle weight was reduced to 53.28% and 51.11% respectively by hindlimb suspension. 2. Soleus muscle mass and relative muscle weight of the training group t day 28 of recovery was restored to the control value. 3. Soleus muscle mass of the training group was greater than that of the cage sedentary group by 6.6% without statistical significance at day 28 of recovery. 4. Relative soleus muscle weight of the traning group was significantly greater than that of the cage sedentary group by 15.79% From these results, it may be concluded that run training during the posthypokinetic period facilitates the recovery of the atrophied soleus muscle mass of rats.