Abstract Fourteen Yorkshire pigs were anesthetized and placed on cardiopulmonary bypass. The hearts were cooled to 15°C and arrested with periodic injections of 50 ml of a cold ( T = 4°C) crystalloid cardioplegic solution containing 35 meq/liter K + for 2 hr. The hearts were then reperfused with warm oxygenated blood for 1 hr. A needle pH electrode (MI-408C, Microelectrodes, Inc., Londonderry, N. H.) was placed into the myocardium and measurements of tissue pH were taken during arrest and reperfusion. Tissue samples were taken during arrest and reperfusion and analyzed for their content of ATP, ADP, AMP, CP, and lactic acid. Blood samples were taken during reperfusion for measurement of CPK. Tissue concentrations of ATP and CP fell during arrest from 4.10 and 6.54 to 3.40 and 1.45 m M respectively. The concentrations of H + and of lactic acid rose during arrest from 2.8 × 10 −8 M and 10.48 m M to 27.5 × 10 −8 M and 18.95 m M , respectively. During reperfusion ATP continued to fall to 2.95 m M while blood CPK rose from 50.81 to 159.49 IU/L. The rise of H + during hypothermic arrest indicates the presence of residual metabolism which may have irreversibly changed the enzyme system for regulating the concentration of tissue ATP. Evidence for this conclusion is given by the escape of CPK into blood during reperfusion.