The systolic time intervals have been studied in nine dogs and one patient undergoing orthotopic cardiac transplantation. The preejection period (PEP) remained within normal limits and appeared essentially unchanged by the operative procedure. The left ventricular ejection time (LVET) however, was abbreviated in the early postoperative period with subsequent return to normal in one week and deterioration in this value coincident with arrhythmias and impending rejection. The use of serial determinations of LVET provides hemodynamic data which, when combined with electrocardiographic and possible biochemical determinations, may be of aid in determining appropriate immunosuppressant therapy.