Novel N-substituted mercaptopropanamide derivatives of the formula: ##STR1## wherein R.sub.1 is mercapto or a group convertible into mercapto when cleaved within the biobody, W is hydrogen atom, an alkyl or an aralkyl, R.sub.2 is an aryl which may optionally have substituent(s), a heterocyclic group which may optionally have substituent(s), or an alkyl which may optionally have substituent(s), X is a cycloalkylene, a cycloalkylidene, or a phenylene which may optionally have substituent(s) or may optionally be fused with other ring, and R.sub.3 is carboxyl or a group convertible into carboxyl when cleaved within the biobody, or a pharmaceutically acceptable salt thereof, and a solid solution of said N-substituted mercaptopropanamide derivative with an amino acid, which have excellent enkephalinase inhibitory activity and are useful for the treatment of mild to moderate pain, and a pharmaceutical composition containing said compounds as an active ingredient, and processes for preparing these compounds.