Compounds are disclosed of general formula (I): ##STR1## wherein R.sub.1 and R.sub.2 each independently represents a hydrogen atom, or an aryl, aralkyl, cycloalkyl, fluoroalkyl or alkyl group, which alkyl group is optionally substituted by an alkenyl group or by a group -OR.sub.7 or by ##STR2## where R.sub.7 and R.sub.8 each independently represents a hydrogen atom, an alkyl, aryl or aralkyl group; or R.sub.1 and R.sub.2 together with the nitrogen atom to which they are attached form a saturated monocyclic 5 to 7 membered ring which may contain a further hetero function (viz--O--, --NH or ##STR3## R.sub.3 and R.sub.4 have the same meanings as R.sub.1 and R.sub.2 and may together form an aralkylidene group;R.sub.5 represents a hydrogen atom or an alkyl or aralkyl group;R.sub.6 represents a hydrogen atom or an aryl or C.sub.1 -C.sub.3 alkyl group;Alk represents an C.sub.1 -C.sub.4 alkylene group optionally substituted at one or more of its carbon atoms by one to three C.sub.1 -C.sub.3 alkyl groups; andX represents an oxygen or sulphur atom,and its physiologically acceptable salts, hydrates and bioprecursors.The indoles (I) may be prepared by combinations of reactions to introduce the desired substituents into suitable intermediates either before or after cyclization to form the indole nucleus. The compounds have selective actions on blood vessels and, in particular exhibit antihypertensive properties. They may be formulated in conventional manner as pharmaceutical compositions.