Previous studies have proved that posttransplant cardiac rejection is associated with changes in endomyocardial electrical impedance in humans. The aim of this study is to develop a safe transoesophageal system able to measure cardiac electrical impedance in transplanted patients and to evaluate the capability of this technique to detect heart graft rejection in humans in order to reduce the number of repetitive biopsies. The impedance measurement is performed using a four wire configuration. We measured the impedance spectrum from 13 kHz to 1 MHz. The oesophageal ECG signal is used to evaluate the position of the catheter in the oesophagus with regard to the patient’s heart. So far measurements have been carried out on a group composed of 11 healthy volunteers (5 men) and 9 cardiac transplanted patients (7 men) with no histological signs of cardiac rejection. Impedance spectrum measurements at different locations in the oesophagus showed a decrease of the impedance magnitude with the increase of the depth of the catheter in the oesophagus (about 1 ohm/cm). The phase value does not show significant changes when changing the catheter’s position in the oesophagus. There are gender differences in the magnitude but not in the phase angle of the transoesophageal cardiac electrical impedance. Transplanted patients with no graft rejection show impedance values similar to that of healthy volunteers, suggesting that the surgical intervention does not affect the impedance measures. The oesophageal approach is safe and was considered for transplanted patients less annoying than the biopsy method. In consequence, if further results show a significant change of impedance in patients with cardiac rejection, a new lessinvasive clinical method to detect graft rejection can be envisaged.