Abstract: Compounds of formula (I), wherein R.sub.1 is selected from the group consisting of an alkyl group, an aralkyl group, an alkenyl group, a cycloalkyl group, in which the alkyl, aralkyl, alkenyl or cycloalkyl group is unsubstituted or substituted with one or more substituents selected from the group consisting of a halogen atom, e.g. Cl or Br, a hydroxyl group or a straight or branched-chain alkoxy group containing from 1 to 6 carbon atoms; and the pharmaceutically acceptable acid addition salts thereof. Such compounds are prodrugs of TRH and are characterized by having a higher lipophilicity than TRH and possessing a high resistance toward degradation by TRH-inactivating enzymes. Such compounds will after administration be converted into TRH.