Objective To investigate changes of collagen Ⅰ and Ⅲ in hypertrophic myocardial tissue of the rat. Methods Sixty SD rats were divided into control groups and model groups. Each group was further subdivided into sub-groups of 1- week, 4-week and 8- week groups,10 rats in each group. The rat of the model group was injected subcutaneously with isop roterenol at the dose of 4mg/(kg.d) for 1 week,4 and 8 weeks, respectively. In the control group,isoproterenol was replaced with saline.The body weight (BW), heart humid weight (HW), left ventricular humid weight (LVW) of each rat were measured. Myocardiac tissues were excised for cryostat section. The myocardiac transverse diameter(MTD),myocardiac section area(MYOR) were measured with an image analysis system. Immunohistochemistry and Western blotting were used to measure the protein concentrations for collagen Ⅰ and Ⅲ. Results The intensities for the collagen Ⅰ and Ⅲ staining were significantly higher in iso-injection 1-week group than those in 4-week group. The results of Western blotting showed that the expression of collagenⅠ and Ⅲ proteins in model groups were higher than that in the control groups（EM>P /EM>0.01）. Collagen Ⅰand Ⅲ contents increase at 1-we