Reverse-turn mimetics and methods relating to the same having the following structure are disclosed: ##STR00001## including pharmaceutically acceptable salts and stereoisomers thereof, wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are as defined herein. Such compounds have utility over a wide range of applications, including use as diagnostic and therapeutic agents. In particular, compounds of this invention, and pharmaceutical compositions containing such compounds, are neurokinin (tachykinin) antagonists. Libraries containing the reverse-turn mimetics of this invention are also disclosed.