Abstract: AIM To explore the effects of Irbesartan on left ventricular hypertrophy and myocardial fibrosis in spontaneously hypertensive rats (SHR). METHODS Sixteen male SHR at 16 weeks of age were randomly divided into treatment group ［receiving Irbesartan 15 mg/(kg·d)］ and SHR-control group, and another 8 WKY rats were used as healthy contro1 group. After 8 weeks, all the rats were killed. The thickness of left ventricle was measured and the left ventricle muscle mass (LVM) was measured to calculate the LVM/BM. Myocardial collage was stained and the interstitial collagen volume fraction (CVF) and the perivascular circumferential collagen area (PVCA) were determined by quantitative morphometry. RESULTS Compared with those in WKY group, the SBP, LVW/BW, thickness of left ventricle, CVF and PVCA were significantly higher in SHR group (P<0.01). They substantially reduced in treatment group compared with those in SHR group (P<0.01). CONCLUSION Irbesartan effectively decreases blood pressure and inhibits myocardial fibrosis and left ventricle hypertrophy.