Objectives: The aim of this study was to find out the effect and action timing of losartan, on myocardial collagen synthesis and ventricular remodeling after myocardial infarction (MI) Methods: 3 groups of 13 rats each, sham operated control (Control, n=13), MI group (MI, n=13), and MI treated with losartan (MI+losartan, n=13)] were compared. Rat hearts were divided into 4 sections– MI site of left ventricle (MILV), right ventricle (RV), septum, and remained non-MI site of left ventricle (NMILV). Collagen volume fraction (CVF) was measured with image pro plus software in the heart tissure stained with picrosirius red(see figure). Also quantification of Hydroxyproline and of procollagen type I and III mRNA using RT-PCR was performed and compared to each groups and sites. Result: 1. CVF of NMILV was 4.9짹1.25% in MI group, 2.4짹1.07% in MI+losartan group (p<0.05). 2. There were no significant differences in CVF of MILV and Septum between MI group and MI+losartan group (p=NS). 3. At 1 week after MI, there were significantly increments of type I and III collagen gene expressions at all site of cardiac tissues in the MI group, and these increments were inhibited significantly by losartan treatment. 4. At 3 weeks after MI, there were no increment of type I and III collagen mRNA at any site in the MI and the MI + losartan group, except MILV sites. Conclusion: Losartan decreases the myocardial collagen synthesis and myocardial remodeling at early stage after MI.