Objective: To investigate potential pathophysiological role of cardiac mast cells accumulation and degranulation on the collagen deposition after coronary microembolization (CME). Methods: CME was induced in miniswine by selective infusion of 15×10~4 microspheres (diameter,45 μm) into the left anterior descending artery groups (CME group,n=8).Some CME-induced animals were pretreated with the MC stabilizer tranilast (50 mg/kg,twice daily),beginning 2 weeks before CME and thereafter throughout the experimental period (CME + tranilast group,n=8),while some animals received tranilast without CME (tranilast group,n=8).Eight sham-operated animals without CME served as controls.After 30 days,the total number of MC and degraunlating MCs and collagen deposition was assessed by histological and electronic microscopy studies. Results: The numbers of total and degranulating MCs and collagen volume fraction (CVF) at day 30 in CME group were significantly higher than those in controls (P<0.01).Treatment with tranilast significantly reduced the numbers of total and degranulating MCs and CVF at day 30 (all P <0.01).There was a significant positive correlation of the CVF with the number of total MCs (r=0.91,P<0.001) and degranulating MCs (r=0.92,P<0.001) over the CME myocardium. Conclusion: MCs accumulation and degranulating contribute to myocardial fibrosis collagen deposition.