The renal clearance of [3H]dextran sulphate by the isolated perfused rat kidney was associated with desulphation of the molecule, as demonstrated by ion-exchange and affinity chromatography of material resident in both glomeruli and urine samples. This process also occurred in vivo. The molecular size distribution of glomerular dextran sulphate in the perfused kidney was indistinguishable from that in the perfusate, and although urinary material was smaller it remained macromolecular. Sulphatase activity was not detected in urine or in the perfusate of perfused kidneys, but was detected in glomerular and non-glomerular cortex fractions isolated by a sieving procedure. The identification of significant biochemical changes to dextran sulphate demonstrates that it does not function as an inert transport probe, and supports the concept of cellular involvement in the process of renal charge selectivity.