A significant reduction in plasma free triiodothyronine (T3) (P< 0.0001) has been observed in patients undergoing open heart surgery. The beneficial effect of T3 would appear to be associated with increased synthesis and utilization of myocardial high energy stores. We have therefore administered T3 (4-10 ug iv) to 10 patients either when difficulty was being experienced in weaning from cardiopulmonary bypass (CPB) support (n = 5), or when myocardial function remained extremely poor (« = 5), despite inotropic and in-traaortic balloon pump support. Mean preoperative NYHA functional class of the 10 patients was 3.2, left ventricular enddiastolic pressure (LVEDP) 20mmHg and ejection fraction (EF) 40%. The mean myocardial ischaemia time was 72 min (range 40-120 min). Within lh of T3 administration the mean plasma free T3 level had risen from 1.03 to 3.56 ug/ml and CPB was discontinued in all 5 cases. Balloon pump support (« = 2) was no longer essential within 3h. At lh, the mean arterial pressure (MAP) had risen from 42 to 78 mmHg, and heart rate (HR) from 90 to 104 beats/min; the left atrial pressure (LAP) had fallen from 30 to 14mmHg, and the central venous pressure (CVP) from 20 to 11 cm H20. (All changes significant at P< 0.0001.) Inotropic support had been significantly reduced or discontinued. To our knowledge, T3 has not been administered previously as an inotropic agent to patients who have undergone cardiac surgery. We believe that T3 may have an important role in the rescue of failing hearts following a period of myocardial ischaemia in patients who have undergone open heart surgery. [Eur J Cardio-thorac Surg (1989) 3:140-145]. © 1989 Springer-Verlag.