There has been little information available on how the use of dextran in vivo influences plasma viscosity and yield stress of blood. Hence, the main purpose of this study is to provide such information when level of red blood cell aggregation in rats was raised to levels seen in normal human blood with administration of Dextran 500. Our results show that plasma viscosity (P < 0.003) and yield stress (P < 0.01) of rat blood increased significantly after dextran infusion. Whole blood viscosity also significantly increased with the dextran treatment over the entire range of shear rates we measured. The viscosity changes at high shear rates after dextran infusion would be due mainly to elevation of plasma viscosity, whereas the changes at low shear rates would be attributed to red blood cell aggregation induced by the dextran treatment.