Inpatients are mostly occupied in bed with restric ted activity, nearly all patient populations are at risk for the occurrence of skeletal muscle atrophy due to decreased level of activity. Restriction of mobility is far greater in pediatric patients compared with adult patients since almost all the activities of daily living is performed by parents or caregivers. It could be assumed that pediatric patients are more vulnerable to skeletal muscle atrophy than adult patients, however, there have been no attempts to reduce the atrophy of developing muscle. Therefore it is important to determine the effect of exercise in developing muscle during decreased activity. The purpose of this study was to determine the effect of periodic weight support during hindlimb suspension on the mass and cross-sectional area of Type I. and II fibers in developing soleus (Type I) muscle. To examine the effectiveness of periodic weight support activity in maintaining mass and fiber size, the hindlimb of young female Wistar rats was sus-pended(HS) and half of these rats walked on a treadmill for 45min/day(15min every 4h) at 5m/min at a 15 grade (HS-WS). After 7days of hindlimb suspension, soleus wet weight was 28. 57% smaller and relative soleus weight was 28. 21% smaller in comparison with con trol rats (p<0.05) Soleus wet weight and relative soleus weight increased by 67.72% and 71.43% each with periodic weight support activity during hindlimb suspension (p(0.01. p<0.005), moreover soleus wet weight and relative soleus weight of the HS-WS rats were greater than those of the control group. No change was observed in fiber type percentage of the developing soleus muscle after 1 week of hindlimb suspension plus weight support activity. Type I and II fiber cross-sectional areas of the developing soleus muscle were 50.45% and 43.39% lower in the HS group than in the control group (p<0.0001), type I and II fiber cross-sectional areas of the developing soleus were 24.49% and 29.93% greater in the HS-WS group than in the HS rats (p<0.0001), whereas Type 1 and II fiber cross-sectional areas of HS-WS group were less than those of the control group. The results suggest that periodic weight support activity can ameliorate developing soleus muscle atrophy induced by hindlimb suspension, even in type II fibers that would not have been expected to be recruited by this type of neuromuscular demand. Clinical experimental study is needed to determine the effect of periodic weight bearing exercise on developing atrophied leg muscle based on these results.