The force-interval relationship of cardiac muscle has been known as not only a fundamental manifestation of beat-to-beat kinetics of intracellular activator calcium which control contractile response but also a potential clinical tool for evaluating cardiac contracile function. In this study were evaluated the force-interval relationship of intact canine left ventricle through mechanical restitution curves by plotting contrctile responses to varying steady state, extrasystolic and postextrasystolic intervals, and compared the force-interval relationships of intact canine left and right ventricles quantitatively. Effects of localized myocardial ischemia on the left ventricular force-interval relationship and relaxtion function were also evaluated 30 minutes after ligating proximal left anterior descending coronary artery through observing contractile and relaxtion responses to various intervals. 1) Mechanical restitution curve of left ventricle showed that left ventricular dp/dt max responses rose stiffly until plateau level with increasing postextrasystolic intervals, then declined with further increment of postextrasystolic intervals. 2) Mechanical restitution curve of left ventricle shifted leftward and upward with shortening of steady state and extrasystolic intervals, which suggest intracellular calcium kinetics during electrical diastole may operate as a mechanism of the force-interval relationship. 3) Steady state contractile responses remained unchanged but maximal contractile responses increased significantly or contractile reserve in intact left ventricle. 4) Normalized force-interval relationships of left and right ventricle were similar quantitatively, which suggest the force-interval relationship is independent of structural factors in intact canine heart. 5) Occlusion of coronary artery lowered absolute values of left ventricular dp/dt max responses to varying postextrasystolic intervals, but didn't show significant changes of normalized dp/dt max responses, which suggest force-interval relationship be also present in spite of localized myocardial ischemia. 6) Responses of normalized left ventricular dp/dt min to varying postextrasystolic intervals were similar to those of normalized dp/dt max but reduced after coronary artery occlusion in the range above 100% dp/dt max response, which may be used for the detection and evaluation of deranged myocardial relaxation in the left ventricle with localized myocardial ischemia.