This study was designed to clarify the morpholgical changes of cardiomyopathic Syrian hamster, UM-X7.1 with special reference to the endocardial endothelium at stage of heart faliure. The 28 week-old Um-X7.1 and normal Golden hamsters, of the same age were used. The morphological changes were examined with transmission and scanning electron microscopy. The changes of endocardial endothelium were as follows