Aim To observe effect of irbesartan on late ventricular remodeling and ventricular function after acute myocardial infarction(AMI) in rats. Methods After ligating left anterior descending coronary artery,17 surviving AMI male rats were randomly assigned to AMI control group(n=8) and irbesartan group(n=9).Sham operated group(n=8) was selected randomly as noninfarcted control.After 8 weeks of drug therapy by gastric gavage,hemodynamics and left ventricular function were measured with catheterization;infarcted area,left ventricular dilatation index(ventricular cavity area on mid left ventricular cross-section/ventricular area on mid left ventricular cross-section) and sphericity index were measured and calculated using morphological methods;interstitial type Ⅰ and type Ⅲ collagen volume fraction(CVF) in the infracted and noninfarcted zone(IZ, NIZ) were analyzed using polarized light by picrosirius red staining. Results There was no significant difference in infarcted area between the two AMI groups(40.02% vs 44.70%,p<0.05).Compared with the sham operated group,left ventricular (LV) end diastolic pressure(LVEDP),left and right relative weight(LVRW,RVRW),thickness of interventricular septum,LV dilatation index and the CVF of type Ⅰand type Ⅲ in the IZ and NIZ of LV were all significantly increased in the AMI control group(each result was-8.80±4.00 mmHg vs 18.94±6.90 mmHg,1.86±0.08 mg/g vs 2.30±0.26 mg/g,0.47±0.02 mg/g vs 0.57±0.03 mg/g,1.97±0.11 mm vs 2.63±0.46mm,0.21±0.04 vs 0.30±0.05,0.26%±0.12% vs 17.60% ±5.26%,0.17%±0.11% vs 4.22%±3.29%,(0.26%)±0.12% vs 3.68%±0.95%,0.17%±0.11% vs 0.73%±0.26%,p<0.001),while body weight(BW),blood pressure(BP),left ventricular systolic pressure(LVSP),sphericity index(SI) of LV,the left ventricular pressure maximal rate of rise and fall(±dp/dtmax) and their adjustment by LVSP(±dp/dtmax/LVSP) were significantly decreased(each was 507.9±19.5 g vs 392.4±51.8 g,94.82±6.73 mmHg vs 70.64±8.77 mmHg,120.38±10.18 mmHg vs 85.72±15.12 mmHg,1.79±0.25 vs 1.34±0.10,4 997±398 mmHg/s vs 2 080±566 mmHg/s,-4 386±728 mmHg/s vs -1 773±456 mmHg/s,41.58±2.57 vs 23.8±4.35,-36.53±6.12 vs-20.30±3.08,p<0.001).Compared with the AMI control group,LVEDP,LVRW,RVRW,thickness of interventricular septum,dilatation index of LV and type Ⅰ and Ⅲ CVF in the NIZ of LV were all decreased(each was 0.62±9.34 mmHg vs 18.94±6.90 mmHg,1.93±0.11 vs 2.30±0.26,0.46±0.05 vs 0.57±0.03,1.98±0.22 mm vs 2.63±0.46 mm,0.23±0.04 vs 0.30±0.05,0.79%±0.39% vs 3.68%±0.95%,0.37%±0.17% vs 0.73%±0.26%,p<0.05 or p<0.01),while SI,±dp/dtmax and ±dp/dtmax/LVSP were significantly increased in irbesartan therapy group(each was 1.73±0.21 vs 1.34±0.10,3 490±613 mmHg/s vs 2 080±566 mmHg/s,-2 991±458 mmHg/s vs-1 773±456 mmHg/s,36.30±2.90 vs 23.80±4.35,-31.20±2.30 vs-20.30±3.08,p<0.01). Conclusions Irbesartan can effectively decrease collagen deposition in the NIZ, restrain LV dilatation and spherical change,and then improve left ventricular remodeling and LV function in rats.