Many cellular responses are initiated by the binding of an extracellular ligand to receptors at the cell surface. The removal of these ligands, or agonists, would therefore be expected to contribute to the process by which cells recover from stimulation. While dilution in the extracellular medium reduces the concentration of most agonists, many cells have developed specific mechanisms for removing particular ligands. One of the more effective mechanisms is to degrade the extracellular agonist and here we review the production and action of the enzymes responsible for the degradation of a number of these agonists.