Heart transplantation in rats has been preferentially done using anatomical models of non-working beating hearts, connected either to the neck vessels or to the abdominal aorta and vena cava. This non-physiological approach results in an altered coronary perfusion and function. It is unknown if such models influence the immunological response to the graft. Orthotopic transplantation, a desirable procedure in rats, is troublesome to perform due to the difficulties of extracorporeal circulation in small animals. We present here a model of a working heterotopic intra-thoracic heart transplantation functioning as a left ventricular bypass.