Objective:To investigate the ouabains effects on the ultrastructure and function of the rat heart. Methods: Male Sprague-Dawley (SD) rats were treated with ouabain and systolic blood pressure (SBP) were recorded weekly. After 4 weeks, echocardiography was performed, hemodynamic parameters were measured by invasive cardiac catheterization and hanges in heart ultrastructure were analyzed using transmission electron microscopy. Results :After treated by ouabain for 4 weeks, there were no significant differences in the mean SBP of the two groups. However, cardiac systolic and diastolic performances were both worsened with ouabain treatment by echocardiography, left ventricular chamber diameters and wall thickness were significantly increased in the rats of ouabain group. Invasive monitoring indicated that left ventricular systolic pressures (LVSP), rate of pressure development (+dp/dt) and rate of pressure decay (-dp/dt) were significantly attenuated and left ventricular end-diastolic pressures (LVEDP) were increased in ouabain group (P＜0. 05). Disorganization of myofilaments, mitochondrial swelling, disruption and vacuolation, hyperplastic collagen fibers were found in ouabain group by transmission electron microscopy. Conclusion:It is suggested that ouabain induces alterations in cardiac ultrastructure and function, and the effects happened before the increase of blood pressure, which indicates that ouabain might damage rat heart independent of blood pressure.