The present study was designed to determine whether the administration of free radical scavengers, superoxide dismutase (SOD), catalase or dimethylsulfoxide (DMSO) is able to ameliorate ischemia/reperfusion injury in the canine kidney and also ascertain whether or not a relationship exists between oxygen free radicals and membrane-bound Na<sup>+</sup>-K<sup>+</sup>-ATPase activity. In 23 dogs, the vascular pedicle of the left kidney was clamped for 75 min at room temperature. The experimental animals received free radical scavengers for 30 min starting at 2 min prior to reperfusion. Renal tissue specimens were enzyme-histochemically examined regarding the activity of membrane-bound Na<sup>+</sup>-K<sup>+</sup>-ATPase, and a marked reduction just before reperfusion was revealed. The SOD- and the DMSO-treated groups showed a marked recovery of the membrane-bound Na<sup>+</sup>-K<sup>+</sup>-ATPase activity; however, the untreated and the cata-lase-treated groups still demonstrated a marked reduction 1 day after reperfusion. At the same time, widespread acute tubular necrosis in the cortex was observed in the untreated and catalase groups in comparison with the SOD and the DMSO groups. In addition, the SOD and the DMSO groups significantly preserved better renal function. Based on these findings, it was thus concluded that free radical scavengers ameliorate the recovery of depressed membrane-bound Na<sup>+</sup>-K<sup>+</sup>-ATPase activity and ischemia/reperfusion injury in the canine kidney.