Abstract: 5-[1-Hydroxy-2-(substituted-amino)]alkyl-8-substituted-carbostyril derivatives and 5-[1-hydroxy-2-(substituted-amino)]-alkyl-8-substituted-3,4-dihydrocarbost yril derivatives represented by the formulas (Ia) and (Ib) ##STR1## WHEREIN R.sup.1, R.sup.4 and R.sup.5 each represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, with at least one of R.sup.4 and R.sup.5 being an alkyl group, and R.sup.2 and R.sup.3, which may be the same or different, each represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an aralkyl group containing a straight or branched chain alkyl moiety having 1 to 4 carbon atoms or a cycloalkyl group having 4 to 6 carbon atoms, or R.sup.2 and R.sup.3 may, when taken together with the nitrogen atom to which they are attached, form a 5- or 6-membered substituted or unsubstituted heterocyclic ring containing 1 or 2 nitrogen, oxygen or sulfur atoms as hetero atoms, the pharmaceutically acceptable acid addition salts thereof, and a process for preparing the same.