Prolyl oligopeptidase (POP) is a serine protease that cleaves small peptides at the carboxyl side of an internal proline residue. Substance P, arginine– vasopressin, thyroliberin and gonadoliberin are proposed physiological substrates of this protease. POP has been implicated in a variety of brain processes, including learning, memory, and mood regulation, as well as in pathologies such as neurodegeneration, hypertension, and psychiatric disor-ders.