Objective This study was aimed to learn spontaneous lesions in the heart of Wistar rats. MethodsA total of 160 male and female matched Wistar rats were studied. Forty rats were sacrificed at 9,2, 8,4 months of age, respectively. Histopathological changes of the heart were observed and analyzed. ResultsNo obvious pathological changes were detected in 9-month old Wistar rats. Mild pathological changes as small foci of necrosis and degeneration were found in the heart in 2.5% (1/40) of the 12-month old rats. The incidence of cardiomyopathy was 57.5%(23/40)and 100%(40/40)in 18- and 24-month old rats, respectively, and there was subendocardial fibrosis in 2.5% (1/40) of the 24-month old rats. The Masson staining showed that mild collagen fibrosis developed firstly at perivascular area and under valve annulus in a few of 9-month old rats, and increased with increasing age.ConclusionsWistar rat can develop heart disease spontaneously. The incidence rises up with increasing age,mainly cardiomyopathy and occasionally subendocardial fibrosis. Collagen fiber deposition may be closely related to the genesis of rat cardiomyopathy.