The aim of this study was to examine the effect of hindlimb suspension (HS) on the expressions of COL1A2 (type I collagen 2 chain) mRNA and its regulatory factors, Transforming Growth Factor-1,2,3 (TGF-1,2,3), phosphorylated Smad3 and Tumor Necrosis Factor- (TNF-) in rat hindlimb muscles. Forty-eight male Wistar rats (age, 5wk) were randomly assigned to HS for 1,3, 7 and 14 days and control (n=6 for each). During the exposure to HS, COL1A2 mRNA expression decreased in the soleus muscle at day 3 and recovered to control level at day 7. The content of TNF-, one of the negative regulatory factors for COL1A2, increased from day3 until day14. On the other hand, the contents of TGF-1, TGF-3 and Smad3, positive regulatory factors for COL1A2, increased at day7. The in situ hybridization for COL1A2 and the immunohistochemistry of TGF-1 and TNF- revealed their expressions around nerve-related tissues including muscle spindles and connective-tissues sheath. The results indicate that the transcriptional activity of COL1A2 in the soleus muscle initially decreases in response to unloading through an increase in TNF- production, there after it returns towards normal level through activated TGF-/smad pathway.