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Mast cell-derived chymase is implicated in myocardial fibrosis (MF), but the underlying mechanism of intracellular signaling remains unclear. Transforming growth factor-beta 1 (TGF-beta1) is identified as the most important profibrotic cytokine, and Smad proteins are essential, but not exclusive downstream components of TGF-beta 1 signaling. Moreover, novel evidence indicates that there is a cross talk between Smad and mitogen-activated protein kinase (MAPK) signaling cascade. We investigated whether chymase activated TGF-beta 1/Smad pathway and its potential role in MF by evaluating cardiac fibroblasts (CFs) proliferation and collagen synthesis in neonatal rats. MTT assay and 3H-Proline incorporation revealed that chymase induced CFs proliferation and collagen synthesis in a dose-dependent manner. RT-PCR and Western blot assay demonstrated that chymase not only increased TGF-beta1 expression but also upregulated phosphorylated-Smad2/3 protein. Furthermore, pretreatment with TGF-beta 1 neutralizing antibody suppressed chymase-induced cell growth, collagen production, and Smad activation. In contrast, the blockade of angiotensin II receptor had no effects on chymase-induced production of TGF-beta 1 and profibrotic action. Additionally, the inhibition of MAPK signaling had no effect on Smad activation elicited by chymase. These results suggest that chymase can promote CFs proliferation and collagen synthesis via TGF-beta 1/Smad pathway rather than angiotensin II, which is implicated in the process of MF.