This study examined the effect of tanshinone II A (TSN II A) on the cardiac fibrosis induced by transforming growth factor beta 1 (TGF-beta 1) and the possible mechanisms. Cardiac fibroblasts were isolated from cardiac tissues of neonatal Sprague-Dawley (SD) rats by the trypsin digestion and differential adhesion method. The cells were treated with 5 ng/mL TGF-beta 1 alone or pretreated with TSN II A at different concentrations (10(-5) mol/L, 10(-4) mol/L). Immunocytochemistry was used for cell identification, RT-PCR for detection of the mRNA expression of connective tissue growth factor (CTGF) and collagen type I (COL I), Western blotting for detection of the protein expression of Smad7 and Smad3, and immunohistochemistry and immunofluorescence staining for detection of the protein expression of phosphorylated Smad3 (p-Smad3), CTGF and COLI. The results showed that TGF-beta 1 induced the expression of CTGF, COL I, p-Smad3 and Smad7 in a time-dependent manner. The mRNA expression of CTGF and COL I was significantly increased 24 h after TGF-beta 1 stimulation (P < 0.01 for all). The protein expression of p-Smad3 and Smad7 reached a peak 1 h after TGF-beta 1 stimulation, much higher than the baseline level (P < 0.01 for all). Pretreatment with high concentration of TSN A resulted in a decrease in the expression of p-Smad3, CTGF and COL I (P < 0.01). The protein expression of Smad7 was substantially upregulated after pretreatment with two concentrations of TSN II A as compared with that at 2 h post TGF-beta 1 stimulation (P < 0.05 for low concentration of TSN I IA; P < 0.01 for high concentration of TSN II A). It was concluded that TSN II A may exert an inhibitory effect on cardiac fibrosis by upregulating the expression of Smad7, suppressing the TGF-beta 1-induced phosphorylation of Smad3 and partially blocking the TGF-beta 1-Smads signaling pathway.