Background Cardiac mast cell-derived chymase is involved in myocardial fibrosis,but the underlying mechanisms remain unclear. Objective To investigate the effect of chymase on the collagen synthesis and its relationship with transforming growth factor-?1 (TGF-?1)/Smad pathway in rat cardiac fibroblasts (CFs). Methods CFs,from neonatal SD rats,were isolated by trypsinization. The collagen synthesis of CFs was determined by 3H-proline incorporation. The protein expressions of TGF-?1,phosphorylated Smad2/3 (P-Smad2/3) and total Smad2/3 were determined by immunoblotting in CFs. Results Chymase (15,30 and 60 ?g/L) increased the 3H-proline incorporation in a concentration-dependent manner. 30 ?g/L chymase stimulation increased the protein expressions of TGF-?1 and P-Smad2/3 in a time-dependently,while little effect on Smad2/3 protein expression was found. The stimulatory effect of chymase on 3H-proline incorporation elicited by 30 ?g/L chymase was blocked in the presence of TGF-?1 antibody or staurosporine,a P-Smad2/3 inhibitor. Conclusion Chymase promotes collagen synthesis of rat CFs,TGF-?1/Smad might be involved into the signal pathway.