Abstract:In this article, the enzymes of brain and associated tissues that can degrade thyrotropin‐releasing hormone (TRH) and luteinising hormone‐releasing hormone (LH‐RH) are reviewed. As both TRH and LH‐RH are considered to act as neurotransmitters or neuromodulators in the CNS, attention is paid to the subcellular location of the enzymes described and how their topographies and substrate specificities fit them to playing roles as inactivating agents for TRH and LH‐RH or as regulators of intracellular concentrations of TRH and LH‐RH. Consideration is also given to enzymes involved in biotransformation of TRH to secondary metabolites that exhibit biological activity and to enzymes involved in the metabolism of secondary