Abstract: The high serum levels of thyroid hormones in rats were induced by oral administration of thyroid tablets (average dose for each rat ：4mg/d×60d). The serum levels of T₃ and T₄ of the rats were measured,and the myocardial histopathological changes were observed. The results showed that at the end of the administration the serum levels of T₃,T₄ were 0.81±0.11,34.70±0.91 nmol/L respectively,and were higher than those in control animals. Under optic microscope the myocardial fibers were found to be disorder in pattern with vague cross striations and broken longitudinal striations. Fibrosis，fatty degeneration and lymphocyte infiltration were found in the pathological foci. Under electron microscope, most of the myofibrils were found to be broken, the Z - lines disappeared or fused with each other,and the mitochondrias in the myocytes were swelled.