(2S,3R)-3-Amino-2-hydroxy-4-phenylbutyric acid (AHPA) derivatives are used as inhibitors of enkephalinase(s) to augment met5-enkephalin-induced anti-nociception. The derivative (2S,3R)-3-amino-2-hydroxy-4-phenylbutanoyl-L-valine (AHPA-Val) is used as a bestatin analogue.