A 5-isoquinolinesulfonyl derivative of Formula (I): ##STR1## wherein l is zero or one;m and n each is an integer of one to nine;m+n is an integer of at least one;R.sub.1 is a hydrogen atom, a C.sub.1-10 alkyl group, a C.sub.5-6 cycloalkyl group or an aryl group; orR.sub.2 and R.sub.3 each is a hydrogen atom, a C.sub.1-10 alkyl group, a C.sub.5-6 cycloalkyl group, an aryl group or an aralkyl group;R.sub.2 and R.sub.3 are C.sub.1-6 alkylene groups and linked directly or through an oxygen atom to form a 5- to 7-membered heterocyclic ring with the adjacent nitrogen atom; or the ##STR2## wherein R.sub.4 and R.sub.5 each is a hydrogen atom, a C.sub.1-10 alkyl group, an aryl group or an aralkyl group and R.sub.6 is a hydrogen atom, a C.sub.1-10 alkyl group, an aryl group, an aralkyl group, a benzoyl group, a cinnamyl group, a cinnamoyl group, a furoyl group or a ##STR3## wherein R.sub.7 is a C.sub.1-8 alkyl group; and the pharmaceutically acceptable salt thereof; and a process for the preparation thereof. The compounds of this invention have a relaxatory action for vascular smooth muscle and are useful as a vasodilator and a hypotensor.