Two groups of dogs were subjected to a 15-minute period of regional myocardial ischemia by snaring the left anterior descending coronary artery proximal to its first diagonal branch. After release of the snare, the dogs were given either placebo (group 1: n = 7) or triiodothyronine (T3) therapy (group 2: n = 6). The dose of T3 given was 0.2 microgram/kg at 30-minute intervals to a total of six doses. Plasma free T3 level fell significantly during the ischemic period in both groups and continued to fall after reperfusion in group 1. In both groups, cardiac function deteriorated significantly during the period of ischemia and rapidly returned to control level after reperfusion. After 90 minutes of reperfusion, however, deterioration of left ventricular function was observed in group 1 and was significantly worse than in group 2, in which hemodynamic function was maintained and, in fact, improved to levels superior to control. It is suggested that T3 therapy may be worthy of trial in patients in whom reperfusion of the myocardium takes place after a relatively short ischemic period (the "stunned myocardium").