Abstract: A catheter-delivered electrode array is used to three dimensionally map the endomyocardial monophasic action potential (MAP). The catheter is used in a method for endomyocardial MAP mapping and for establishing diagnostic criteria for the cardiac myocyte injury. During allograft rejection, conduction impairment occurs within the atrial myocardium. Abnormalities of the terminal force of the P wave in Lead V1 (ptfV1) and dispersion of corrected atrial repolarization (Ta-TcD) represent interatrial conduction defects that accompanies the rejection process. Three dimensional endomyocardium monophasic action potential mappings are used to directly monitor the pathophysiological changes in cardiac myocytes. Changes in amplitude, duration and morphology of the action potential were recorded and used to detect the early rejection of the transplant organ with high sensitivity and specificity.