Objectives: This study explores the mechanism of tanshinone IIA (TSN)-mediated inhibition of myocardial fibrosis by investigating the effect of TSN on transforming growth factor beta 1 (TGF beta 1) signal transduction in rat cardiac fibroblasts (CFs). Materials and Methods: CFs were isolated from neonatal Sprague-Dawley rats by trypsin digestion and differential adhesion and stimulated with 5 ng/mL TGF 1 and TSN (10(-6), 10(-5), or 10(-4) mol/L). The expression of fibronectin (FN) mRNA in the CFs was determined using reverse transcriptase-polymerase chain reaction and the protein expression of FN and Smads in CFs was detected using Western blot. The intracellular expression and localization of Smads in the CFs were analyzed using immunocytochemistry. Results: TGF beta 1 induced the expression of FN and Smads in a time-dependent manner. At the end of the culture treatment, the mRNA expression of FN and the expression of phosphorylated Smad2/3 (p-Smad2/3) increased significantly (P < 0.01). TSN pretreatment (10(-5) and 10(-4) mol/L) reduced the expression of FN and p-Smad2/3 (P < 0.01) following TGF beta 1 stimulation and led to a significant decrease in the nuclear staining intensity and a positive rate of p-Smad2/3 (P < 0.05 and P < 0.01, respectively). Conclusion: The inhibitory effect of TSN on myocardial fibrosis may be associated with its inhibition of TGF beta 1-induced Smad2/3 phosphorylation and p-Smad2/3 nuclear translocation, which blocks the TGF beta 1/Smad signaling pathway in CFs.