Selected patients with proteinuria will be asked to participate in studies of glomerular capillary wall permselectivity calculated from the fractional clearances of the endogenous proteins, albumin and IgG and from the fractional clearances of polydisperse neutral dextran with effective molecular radii ranging from 20-60 angstroms. To achieve this, low molecular weight dextran will be administered by slow IV injection immediately following the inulin or iothalamate and/or PAH priming doses.