In the past, assessment of cardiac function has centered on techniques that examined the heart's action as a pump. Hemodynamic analysis of cardiac function within the framework of the Frank-Starling relation is valuable when directional changes in the performance under stress of the diseased ventricle are compared with the normal. Recently, principles of skeletal muscle physiology have been extended to the myocardium and applied successfully to examination of the function of the intact heart. Angiography in patients revealed that specific alterations in the pattern of contraction occur in patients with left ventricular disease, principally a decrease in extent and velocity of fiber shortening, associated with decrease in the extent of reduction of ventricular wall tension during contraction. Estimates of contractile state based on the tension-velocity-length relation determined angiographically sensitively quantify the myocardial contractile state in patients, even during abnormal ventricular loading. A second useful method for assessment of cardiac function in patients consists of relating the first derivative of the ventricular pressure to the simultaneously occurring isovolumic pressure. A third promising method measures pulmonary arterial and aortic blood flow velocity by a cathetertip velocity probe. This instrument uses the principle of the external electromagnetic flow meter; its potential applicability to assessment of myocardial function has been shown. Use of the cardiac output response to exercise in the assessment of cardiac function is reviewed. Measurement of the cardiac output when exercise has lowered the mixed venous oxygen saturation to 30% is useful, reliably and sensitively distinguishing performance of the patient with mild or moderate impairment of cardiac function from normal.