The anatomic constraints and design parameters for a heart prosthesis have not yet been defined in heart transplant recipients (i.e., the population most eligible for total artificial heart implantation). The parameters regarding anatomic constraints were measured in 26 consecutive patients undergoing orthotopic heart transplantation (median body surface area 1.9 m2) after cardiectomy. A full-sized contour model of the cylindric total artificial heart (diameter 97 mm; width 81 mm) was inserted into the pericardial cavity to decide the pump configuration and to verify its fit. The dimensions of this model were based on the miniature electromechanical total artificial heart that is currently under development. Fit was found to be adequate in most of the cases with no identifiable compression of adjacent vascular structures. The median intraoperative measurements that define pericardial constraints for a heart prosthesis were pericardial length (130 mm), width (160 mm), and depth (140 mm). We also took measurements from the excised hearts, which should provide a useful reference for other prosthetic devices. The current dimensions of our implantable total artificial heart were found acceptable for orthotopic implantation. Length of the pericardium and cardiothoracic ratio were identified as variables related to adequacy of fit.