Summary:This study demonstrates that there is only small conformational dependence for the fractional clearance, as studied in the isolated perfused kidney, of random coil dextran as compared to the spherical Ficoll when these probes are standardized on the basis of effective hydrodynamic radii by gel exclusion chrotography. This is particularly for molecules with the same effective hydrodynamic radii of 3.6 nm as albumin. the influence of osmotic stress exerted by plasma concentrations of albumin is shown to be minor for dextran. These studies demonstrate that there must be some other factor apart from conformation that accounts for the relatively low fractional clearance of albumin compared with dextran of equivalent hydrodynamic radius.