A number of peptide hormones have been shown to undergo receptor-mediated endocytosis (RME). RME involves the internalization of receptor-ligand complexes followed by delivery to an intracellular compartment, the endosome, from which ligands or receptors can be delivered to lysosomes or other cellular destinations. Vasopressin, a peptide hormone that plays a role in kidney and vascular physiology, has recently been demonstrated to undergo RME in LLC-PK1 and A10 cells, which express V2- and V1-type vasopressin receptors, respectively. Fluorescent vasopressin analogues are internalized by RME from the basolateral surface of polarized LLC-PK1 cells. The precise role of RME in vasopressin action is uncertain, but it is likely that it is involved in the desensitization of target cells by altering the number of cell surface vasopressin receptors. Alterations in the rate of RME may alter the response of the cell to vasopressin. Fluorescent and biotinylated vasopressins are useful tools for the study of this process.