The after effects of renal ischemia were studied in hypoxia-adapted rats. It was felt that chronic hypoxia animals which had already adapted to a low oxygen level might be more tolerant of renal ischemic insult; however, chronic hypoxia is always accompanied by polycythemia, which may cause severe RBC trapping and consequently might enhance renal damage after renal ischemia. Chronic hypoxic rats were prepared by exposure in an altitude chamber 15 h per day for 4 weeks. The plasma sodium, potassium, urea, and creatinine levels were determined to compare the changes in these parameters between the baseline and 3 h afterα45-min occlusion of both renal arteries in 12 sea-level (SLB) controls and in 12 chronic hypoxic (CHB) and II chronic hypoxic plus RBC pheresis (to reduce hematocrit: CH + P) rats. From the parameters measured, the CHB rats were found to be more tolerant of renal ischemia. However, this was not the case in the rats with pheresis. It is concluded that after chronic hypoxia, some humoral factors in the plasma may play an important role in reducing the renal damage after ischemic insult.