A simple method has been developed whereby polymers of polyvinylpyrrolidone and dextran can be visualized at the ultrastructural level by postfixing tissue with osmium tetroxide partially reduced by potassium ferrocyanide. Further enhancement of polymer electron opacity is obtained by counterstaining thin sections with bismuth subnitrate chelated by alkaline tartrate. These tracers remain adequately dispersed in serum and, depending on molecular size and concentration, are easily detected at the ultrastructural level within vascular lumena, basement lamina, apical vacuoles of proximal and distal tubules and endothelial vesicles. This technique offers the potential of studying nephron and cell permeability with families of tracers, the members of which are similar in chemical affinities, charge density and molecular shape.