We measured blood viscosity in rats 24 h after induction of acute renal failure by glycerol or HgCl<sub>2</sub> injections. The blood viscosity values of rats with acute renal failure were significantly higher than those of controls at any shear rates. The mean values of plasma fibrinogen in animals with glycerol – (302.5 ± 35.1 mg/100 ml) or HgCl<sub>2</sub> -(342.1 ± 15.9 mg/100 ml) induced acute renal failure were significantly elevated compared to control levels (169.6 ± 16.7 mg/100 ml). We believe that the increased blood viscosity in acute renal failure was primarily due to high concentration of plasma fibrinogen. The elevated blood viscosity may affect capillary microvascular circulation of glomerular capillary beds.