Red blood cell (RBC) deformability is necessary for maintenance of normal oxygenation in microcirculation. To evaluate oxygen utilization in the early stage of endotoxin shock, we observed RBCs from a rheological point of view in mesenteric microcirculation in rats. Endotoxin shock was induced by Shwaltzman reaction. The velocity of RBC decreased from 1.33 mm.sec-1 to 0.55 mm.sec-1 at 60 min after the provoking injection. RBC could not change its shape in capillaries. In contrast, the mean blood pressure remained at 110 mmHg throughout this observation. In hemorrhagic shock the deformability of RBC in capillaries was not impaired even though mean blood pressure decreased to 43 mmHg. In vitro, we examined the index of RBC deformability with or without endotoxin by the modified Reid's method. The index was decreased significantly by endotoxin. We conclude that impaired deformability of RBC may be a factor of insufficient oxygen utilization in the early stage of endotoxin shock.