The effects of the presence or absence of calcium in the cardioplegic perfusate were studied utilizing the isolated blood perfused dog heart preparation. Hearts were subjected to two hours of arrest at 27 degrees C followed by 90 minutes of normothermic reperfusion. Perfusates with or without 2.52 mM calcium chloride were delivered at 15 minute intervals during arrest at a perfusion pressure of 100 mm Hg. Both calcium and calcium free perfusates resulted in decreases in tissue calcium concentration measured at the end of arrest. Tissue magnesium concentrations did not change with either perfusate. Coronary vascular resistance was increased with early perfusions in the calcium perfused group relative to the calcium free group. Systolic and diastolic performance, high energy phosphate values, coronary blood flow and myocardial oxygen consumption were altered by the arrest-reperfusion sequence but no differences between calcium and calcium free groups were seen. Thus, no evidence for deleterious effects of calcium free perfusion was present after two hours of global cardiac arrest followed by reperfusion. The data indicate the absence of the calcium paradox under conditions simulating clinical cardioplegia and supports clinical data indicating that the addition of calcium to the cardioplegic solution is unnecessary.