Objective To observe the histological architecture of reticular fibers and agerelated changes during development of rats, in order to establish abase for probing myocardial tissue remodeling characteristics. Methods Using technique of paraffin section, Gomori silver staining, the myocardial tissue of fetal rat (pregnant about 19d), newborn rat (9d), young rat (8m) and old rat (14m) were observed with light microscopy. The expression differences of Ⅲ type collagen were detected by Western blotting. The reticular fibers was quantitated with image analysis system, and the data were processed with SPSS statistical software. Results The reticular fiber could be divided to thin and thick fibers that encircled myocytes. The reticular fibers around myocyte showed thin string or absence in fetal rat. As for the newborn rat, the reticular fibers around myocytes gradually increased and formed continuous membranes, and some spaces that were full of branch of reticular fibers appeared between myocytes. The area percentage of reticular fibers was gradually changed. Ⅲ type collagen (reticular fiber) expression changed with aging, and sharply increased from newborn to young rat.Conclusion The reticular fibers are composed of thick or thin fibra sheaths around myocytes, and the branchs of reticular fibers are full of the space between myocytes or between myocytes and interstitium, and the expression of Ⅲ type