This study was designed to evaluate the effect of reperfusion after 60 min of ischemia on Ca2+ and Ba2+ exchanges in the newborn and adult myocardium. All studies were performed utilizing the isolated arterially perfused septal preparations of newborn and adult rabbit hearts that were maintained at 27, 33, or 37 degrees C and paced at 90 beats/min. After stabilization of mechanical function, 60 min of global ischemia was induced and then followed by 60 min of reperfusion. Tissue 47Ca2+ or 133Ba2+ uptake was measured by a juxtaposed gamma probe. 133Ba2+ uptake studies were utilized to determine sarcolemmal permeability to divalent cations. During reperfusion, there was an increase in the rate of Ca2+ and Ba2+ uptake at all temperatures in the adult and only at 33 and 37 degrees C in the newborn. The increase in the rate of divalent cations was proportionally related to the muscle temperature, and the increase in the newborn was significantly less than that in the adult. The net increase in tissue Ca2+ was inversely related to the recovery of myocardial mechanical function and ATP content after reperfusion. These data suggest that 1) an increase in sarcolemmal permeability to divalent cations on reperfusion may be responsible for the increase of Ca2+ uptake and this increase in the newborn was significantly less than in the adult, and 2) this increase in Ca2+ uptake is associated with myocardial ATP depletion.