Rates of synthesis and degradation of heart protein were measured during 30 or 60 min of myocardial ischemia and during the 30 or 60 min following these ischemic periods. During ischemia, rates of protein synthesis and degradation were reduced. Resumption of control rates of coronary flow after 30 min of ischemia resulted in complete restoration of creatine phosphate and partial recovery of ATP (75%), developed pressure (79%), and cardiac output (80%). After 60 min of ischemia, restoration of flow completely restored creatine phosphate but resulted in poor recovery of ATP (57%), developed pressure (26%), and cardiac output (63%). During the recovery phase, rates of protein synthesis and degradation of protein to free amino acids were the same as in hearts that had been perfused for a comparable period as working aerobic preparations. These findings were consistent with inhibition of an initial step in proteolysis during the period of ischemia followed by return to control rates of degradation when oxygen delivery and energy levels were restored.