Hyperthermia (39.5 degrees C) worsens experimental ischemic acute renal failure (ARF). We assessed whether it does so by affecting the ischemic and/or reperfusion injury phase and if its influence is mediated through changes in kidney ATP content and xanthine oxidase-mediated oxidant stress. Rats were subjected to 25 minutes of renal pedicle occlusion and hyperthermia was imposed during ischemia alone, reflow alone (0 to 30, 30 to 60, and 60 to 90 minutes), ischemia + reflow, or without ischemia. Hyperthermia's effects on ischemic/reperfusion adenylate pools lipid peroxidation (malondialdehyde), and the severity of ARF were assessed in comparison with normothermic ischemic controls. Hyperthermia confined to ischemia profoundly worsened ARF whereas during immediate reflow (0 to 30 minutes) hyperthermia had only a mild ARF-potentiating effect. During late reflow (greater than 30 minutes) or in the absence of ischemia, hyperthermia caused no damage. Hyperthermia had only a brief negative impact on ischemic ATP content, just slightly lowering it during the first 5 minutes of ischemia. Nevertheless, much greater ischemic damage resulted, reflected by increased proximal tubular brush border membrane sloughing at the end of vascular occlusion. Hyperthermia imposed only during reflow did not affect ATP concentrations. Hyperthermia increased end-ischemic purine base concentrations by 10% due to increased ATP degradation. However, reperfusion lipid peroxidation did not result and xanthine oxidase inhibition (by oxypurinol) conferred no protection. CONCLUSIONS (a) Hyperthermia worsens ARF predominantly by affecting ischemic, not reperfusion, injury; (b) xanthine oxidase is not an important mediator of hyperthermic-ischemic ARF; and (c) hyperthermia has a quantitatively trivial impact on ischemic ATP levels. This suggests that hyperthermia principally worsens ARF by magnifying the consequences of energy depletion (e.g., membrane damage) more than by worsening energy depletion, per se.