Abstract: AIM: To explore the effect of growth factor-β1 (TGF-β1)/Smad pathway on the proliferation of chymase-induced rat cardiac fibroblasts (CFs). METHODS: CFs from neonatal Sprague Dawley rats isolated by trypsinization were cultured. DNA synthesis of the CFs was determined by ［3H］-TdR incorporation and protein expressions of TGF-β1, p-Smad2/3, Smad2/3 and Smad7 in CFs were determined by Western blot. RESULTS: Chymase increased the ［3H］-TdR incorporation of CFs in a concentration-dependent manner. The ［3H］-TdR incorporations induced by 15, 30 and 60 μg/L chymase were (319±29), (372±43) and (401±47) cpm/well, respectively, significantly higher than in control groups (252±35) cpm/well, P<0.01). Chymase also increased the expression of TGF-β1 protein in a concentration-dependent manner. TGF-β1 protein expressions in 15, 30 and 60 μg/L chymase groups were 0.968±0.069, 1.782±0.058 and 2.656±0.085, respectively, significantly higher than in control groups (0.333±0.023, P<0.05 or P<0.01). Also, 15, 30 and 60 μg/L chymase obviously increased the p-Smad2/3 expression and decreased Smad7 protein expression in a concentration-dependent manner as compared with control groups (P<0.05 or P<0.01) but had no effect on the protein expression of Smad2/3. CONCLUSION: Chymase promotes proliferation of rat CFs and the increase may be related to the activation of TGF-β1/Smad signaling pathway.