To determine the protective effects of amino acids on the ischemic and reperfused myocardium, an experimental study was carried out. Rats were starved for 20 days, then separated into the following three groups: group I: control, group II: glutamate and aspartate were administered before aortic cross clamping for 5 minutes, and added to the cardioplegic solution, group III: glutamate and aspartate were given during the initial 15 minutes of reperfusion. After 25 minutes of equilibration, the hearts were made ischemic at 35.5 degrees C for 45 minutes, and arrested with high potassium cardioplegic solution immediately after aortic cross clamping. The recovery of cardiac output in group II was significantly (p less than 0.05 vs group I & III) better than that in groups I and III, with no difference between groups I and III. Tissue analysis for high energy phosphate compounds revealed that the levels of ATP and total adenine nucleotides were significantly (p less than 0.05 vs group I & III) higher in group II. Lactate output during the initial three minutes of reperfusion was decreased to the greatest extent in group II. Metabolic studies involving the examination of oxygen utilization also revealed no differences between groups I and III. Thus, addition of glutamate and aspartate before and during aortic cross clamping is effective for reducing ischemic damage, however, the administration of these amino acids during the reperfusion is without significant effects.