Factors affecting the contraction of isolated cardiac muscle, preload, afterload and contractility, in addition to the heart rate, are also the primary determinants of the cardiac output in the intact ventricle. In the intact heart, ventricular end-diastolic wall stress is analogous to the preload and within physiologic limits ultimately determines the resting length of the sarcomeres in the ventricular wall. The relationship between the end-diastolic pressure and the stroke volume can be used to describe ventricular function and the relationship between stroke volume and end-diastolic volume represents the ejection fraction. The stroke volume, in turn, is inversely proportional to the afterload during the ejection phase. Since the afterload decreases during the ejection phase, the contraction of the intact ventricle can be characterized as being auxotonic. The contractility is reflected by the maximum force development as well as rate of shortening. Changes in the contractility represent changes in the ventricular function which are independent of the preload and afterload. Increasing the heart rate has little effect on the stroke volume despite an increase in the contractility. The relationship between pressure, flow and radius can be used to assess ventricular function since this nearly approximates that of the force-velocity-length relationship. The stiffness and stiffness constant of the muscle can be derived from the stress-strain relationship while the stiffness of the ventricular chamber may be described by the relationship between pressure and volume. The contractility indexes of the isovolumic phase are, to a certain degree, indicative of the force-velocity-length relationship of the heart muscle. The contractility indexes of the ejection phase are afterload-dependent but correlate well with the contractility of the myocardium. Construction of a line connecting points of end-systolic pressure-volume values corresponds with that of the force-velocity relationship, the slope of which may accurately reflect the ventricular contractility. None of these indexes, however, completely represents the force-velocity-length relationship of the intact heart. At present, the best measurements of contractility combine use of various parameters as well as data obtained from a series of contractions.