Mitochondrial Ca2+ accumulation and mitochondrial respiratory dysfunction have been observed after renal ischemia. The present study examined the effects of ischemia and reperfusion on cellular energy production and mitochondrial Ca2+ transport after 50 min of bilateral renal artery and vein clamping in the anesthetized rat. Prior to reflow, tissue ATP and total adenine nucleotide levels were severely reduced. These nucleotide levels recovered towards normal but remained lower than control values throughout 24 h of reperfusion. Energy-linked mitochondrial Ca2+ uptake was unmeasurable, mitochondrial Ca2+ efflux was increased and the mitochondria were unable to maintain a steady-state free Ca2+ concentration prior to reflow. Three hours of reperfusion was associated with a normalization of mitochondrial Ca2+ uptake, release, and steady-state buffering. However, progressive deterioration subsequently occurred in these processes. Thus the mitochondrial Ca2+ accumulation previously observed during the later stages of postischemic reperfusion (3-24 h) is due neither to an increase in the rate of active Ca2+ uptake nor to a decrease in the rate of Ca2+ release. The present results therefore suggest that passive mitochondrial Ca2+ accumulation may occur during the later stages of reperfusion, probably due to a progressive increase in the cytosolic Ca2+ concentration.