The invention relates to new N-substituted bis(hydroxymethyl)-methyl-isoquinoline derivatives of the formula (I), ##STR1## wherein R.sup.1 and R.sup.2 represent hydroxyl or alkoxy having from 1 to 6 carbon atoms,the dotted line is an optional double bond in the 1,2-position,R.sup.3 is alkyl having from 1 to 6 carbon atoms or aralkyl containing from 1 to 4 carbon atoms in the alkyl moiety, or--if there is a single bond in the 1,2-position--represents a group of the formula ##STR2## in which X is oxygen ifR.sup.4 is alkyl having from 1 to 5 carbon atoms, alkoxy having from 1 to 4 carbon atoms, hydroxyl, phenyl or substituted phenyl; orX is oxygen or sulfur ifR.sup.4 is an --NH.sub.2, --NH--C.sub.1-4 -alkyl, --NH--C.sub.3-6 -cycloalkyl, --NH-phenyl or --NH--C.sub.1-4 -alkyl-phenyl group; andR.sup.5 and R.sup.6 independently stand for hydrogen, alkyl having from 1 to 4 carbon atoms or phenyl, with the proviso that at least one of them is other than phenyl,and salts thereof. Processes for the preparation of these compounds and pharmaceutical compositions are also within the scope of the invention.