The effect of normothermic ischemia and ischemia/reperfusion on the function of cardiac sarcoplasmic reticulum (CSR) was investigated using a modified Langendorff perfusion of isolated rat hearts. The function of the CSR was assessed by the oxalate-supported Ca2+uptake rate of ventricular homogenates. The contribution of the ryanodine-sensitive portion of the CSR was determined by using 20 μM ruthenium red or 625 μM ryanodine to close the CSR Ca2+release channel. The Ca2+uptake rate of the CSR decreased progressively with increasing duration of ischemia, but this depression was much less when uptake was assayed in the presence of ryanodine. The depression in CSR Ca2+uptake preceded ischemic contracture. Ryanodine and ruthenium red stimulated uptake almost equally in control hearts, but ruthenium red was much less effective than ryanodine after ischemia. This difference could not be overcome by increasing the ruthenium red concentration. These results confirm the suggestion that the Ca2+release channel is inappropriately opened after ischemia. The CSR uptake rates were almost completely restored at 15 minutes of reperfusion after 5 and 10 minutes of ischemia but were only partially restored after 15 minutes of ischemia. At reperfusion, mechanical function (end-diastolic pressure and peak systolic developed pressure) was markedly depressed after only 15 minutes of ischemia. The degree of “stunning” correlated well with the depression of CSR function in individual hearts. The decreased Ca2+uptake of the CSR was not due to a buildup of ADP in the homogenates. These results suggest that the reversible ischemic damage of the CSR may be due to a reversible regulation of CSR function and that this regulation may be involved in the causal cascade of events that give rise to postischemic stunning.