Objective To define the relation ship between the histological architecture and the proportion of collagen Ⅰand Ⅲ in the rat myocardium. Methods Immunohistochemistry, immunofluorescence double labeling and Western blotting were used for studying construction characteristics of collagen Ⅰand Ⅲ in 10 adult rat hearts. Collagen Ⅰand Ⅲ were quantified by image analysis system. Results Collagen Ⅰ formed mainly thick fibers which were mainly distributed around bundles of myocardial cell and tunica adventitia of cardiac vessels, whereas collagen Ⅲ formed thin collagen fibers which were around bundles myocardial cells and tunica adventitia of the vessels. However, both thick and thin collagen fibers contained various proportions of collagen Ⅰ and Ⅲ. Conclusion The Collagen network in the rat heart was composed of thick and th