To investigate the mechanism underlying postischemic cardiac dysfunction (myocardial stunning), contractility and adenine nucleotide metabolism were studied in three groups of isolated perfused rabbit hearts (control, ischemic, and reperfused), whereas Ca2+uptake by the sarcoplasmic reticulum (SR) was measured in homogenates obtained from them. The hearts were Langendorff-perfused under constant pressure with Krebs-Henseleit solution at 37°C. Global normothermic ischemia was produced by closing the perfusion line. In the reperfused group, after 15 minutes of ischemia, Krebs-Henseleit solution was perfused for 10 minutes. Developed left ventricular pressure (control, 104±6.3 mm Hg) and left ventricular dP/dt (2,063±256.6 mm Hg.sec−1) were significantly decreased in reperfused hearts (left ventricular pressure, 78±5.9 mm Hg; left ventricular dP/dt, 1,339±216.3 mm Hg.sec−1). Myocardial ATP content (control, 13.6±0.98 μmol/g dry wt) decreased during ischemia (4.5±1.23 μmol/g) but was restored to control level on reperfusion (11.8±0.68 μmol/g). Maximum velocity of Ca2+uptake by the SR (Vmax) (control, 49.3±2.54 nmol.min−1.mg−1) was significantly depressed by ischemia (36.3±1.94 nmol.min−1.mg−1) but was restored to the control value after a 10-minute reperfusion (45.3±0.79 nmol.min−1.mg−1). Apparent dissociation constantKCaand the Hill coefficient for Ca2+uptake were not different between control, ischemia, and reperfusion. To test for the possible role of the SR Ca2+-release channel in the effect of ischemia and reperfusion, we measured Ca2+uptake after incubation of homogenates with 610 μM ryanodine. The changes in Vmaxcaused by ischemia and reperfusion were qualitatively similar to those observed in experiments without ryanodine (76.3±5.08, 54.0±5.08, and 69.7±2.82 nmol.min−1.mg−1for control, ischemia, and reperfusion, respectively). These results suggest an effect of ischemia on the SR Ca2+pumping without an effect on the Ca2+-release process. The recovery of Ca2+uptake during reperfusion indicates that neither an altered uptake of Ca2+by the SR nor an abnormal function of the release channels is the major cause of myocardial stunning.