Objective To approach the effects of rosiglitazone treatment on transforming growth factor-β1/Smad(TGF-β1/Smad) signaling pathways in Otsuka Long-Evans Tokushima Fatty(OLETF) rats. Methods Experimental rats were divided into three groups: OLETF group, rosiglitazone-treated group(OLETF/L group) and Long Evans Tokushima Otsuka rats (LETO group) as control, 8 rats in each group. The expression of TGF-β1, Smad2, Smad3, Smad7, fibronectin(FN), and collagen type Ⅲ in rat lungs were examined by immunohistochemistry. Results The expressions of TGF-β1, Smad2, Smad3, FN and collagen type Ⅲ increased in OLETF rat lungs than those of LETO rats(EM>P/EM><0.01), while the expression of Smad7 decreased in OLETF group(EM>P/EM><0.01). The expressions of TGF-β1, Smad2, Smad3, FN and collagen type Ⅲ decreased in OLETF/L group than those of the OLETF group(EM>P/EM><0.01), and the expression of Smad7 increased in OLETF/L group(EM>P/EM><0.01). Conclusion