In an effort to understand how ischemia and reperfusion might affect blood cell rheology, erythrocyte and leukocyte filterability was measured in blood samples collected from the renal vein from rabbits without previous ischemia or during reperfusion after sixty minutes of ischemia. A gradual decline in blood cell filterability was observed during continuous sampling in both circumstances. However, during reperfusion after ischemia, the filterability of erythrocytes and leukocytes was even more reduced as compared with that of control animals. This ischemia-induced reduction in filterability could be partly prevented by pretreatment with superoxide dismutase and catalase. An impaired cellular filterability induced by oxygen free radicals produced during recirculation after ischemia might contribute to the local erythrostasis found in such kidneys.