The invention relates to the compounds of formula (I): ##STR1## in which: R.sub.1 represents alkyl, phenyl, naphthyl, pyridyl or thienyl group, each phenyl, naphthyl, pyridyl or thienyl optionally being substituted,R.sub.2 represents a hydrogen atom or a substituted or unsubstituted alkyl, substituted or unsubstituted phenyl, cycloalkyl, piperidino or substituted or unsubstituted amino group,X represents CO or SO.sub.2,R.sub.3 represents hydrogen or alkyl,R.sub.4 represents alkyl, substituted or unsubstituted phenyl or trihalomethyl, or else R.sub.3 and R.sub.4 form, together with the carbon atoms which carry them, cyclo(C.sub.3 -C.sub.7)alkenyl,A represents phenyl, naphthyl or pyridyl ring, each phenyl, naphthyl or pyridyl ring optionally being substituted,their isomers, the corresponding quaternary ammonium salts of the piperidine and their addition salts with a pharmaceutically acceptable acid, an medicinal products containing the same are useful as antagonists of NK.sub.1 receptors.