1. Definition. The isolated perfused heart according to Langendorff is a preparation which is easily obtained from all warm-blooded animals. The heart is usually perfused at constant pressure, but frequently at constant flow, mostly with oxygenated saline solutions. The ventricles do not fill with the perfusate and therefore do not perform pressure-volume work. If, however, an intraventricular balloon is inserted, the ventricle can contract isovolumetrically. 2. Measuring parameters. Mechanical parameters of the working myocardium (contractile force, volume, ventricular diameter), mean coronary flow, bioelectrical parameters (EKG, monophasic injury potentials), and cardiac rhythm can be measured and recorded. 3. Function tests. The intact function of the working myocardium and the coronary vessels should be examined using various simple tests regarding the myocardium, the smooth musculature, and the endothelium of coronary vessels. 4. Possible applications. In the Langendorff-heart of normal or pretreated animals, inotropic, chronotropic, antiarrhythmic or vasoactive substances can be investigated in the steady state or by means of specific stress tests. In addition, the preparation is particularly suitable for biochemical studies of myocardial metabolism. 6. Advantages. Due to the technical ease with which a variety of parameters can be measured, a single heart can provide maximum information about the condition of the myocardium and the coronary vessels. Thus, it is not surprising to find that 90 years after its first description, the isolated perfused heart according to Langendorff continues to be utilized.