Ten postcardiac surgical patients with acute renal failure (ARF) were infused with inulin and dextran 40. Plasma and urine were then submitted to gel-permeation chromatography to ascertain the apparent fractional clearance profile for the dextrans. Compared to normal volunteer controls, the fractional clearance profile was substantially elevated for dextran molecules in the Einstein-Stokes radius (r) range 20-40 A. For the smaller molecules (r = 20-28 A), fractional dextran clearance in ARF was frequently in excess of unity. A simple mass conservation model which assumes that the "true" fractional dextran clearance profile for the glomerulus (in Bowman's space) in ARF is the same as that for normal controls, when applied to the experimental observations, revealed that in ARF, on the average, 50% of filtered inulin is lost by tubular backleakage. Furthermore, the model permitted an estimate of the permeability properties of the damaged tubular wall. This indicated tubular permeability not unlike that of the normal glomerulus to dextran molecules with r less than 30 A, but relative impermeability to larger dextran molecules.