The Isolated Perfused Heart System, as originated by Oscar Langendorff at late 19th century, is a predominant technique in pharmacological and physiological research. We use Radnoti Isolated Working Heart System that may be used both in Langendorff and Working Heart modes. In Langendorff method, aorta is cannulated and perfused with Krebbs- Henseleit solution. A constant hydrostatic pressure or desired pressure by peristaltic pump may be used. A left ventricular pressure measurement is performed to evaluate the effects of drugs used on ventricular function e.g. pressure and heart rate. In Working Heart mode, the heart is perfused through the left atria. Perfusate enters the left ventricle and is ejected out of the aorta, mimicking normal heart function. Both preload (left atria) an afterload (left ventricle) pressures and heart rate may be recorded with ECG, heart force and heart temperature measurements. Both constant pressure and constant flow models for isolated heart as well as both recirculating and non-recirculating modes are provided. The system is at its best in screening new drug candidates.