The design and use of an enzyme inhibitor as a pharmacological agent must take into consideration a number of important factors. These include the potency of the inhibitor, the specificity of the inhibitor, the toxicity of the inhibitor, and the ability of the inhibitor to be delivered to the desired site of its action. This article will primarily focus on the progress made in the design of specific neuropeptide degrading peptidase inhibitors without taking into account the toxicity or mechanism by which the compound may be delivered to a particular site of action. Emphasis is placed on those neuropeptide degrading peptidase inhibitors which either have promise as pharmacological agents themselves or which may serve as models for the future development of pharmacologically useful compounds.