Abstract: Indole derivatives are disclosed of formula (I): ##STR1## wherein R.sub.1 represents H, alkyl or alkenyl;R.sub.2 represents H, alkyl, alkenyl, cycloalkyl or phenyl or phenyl alkyl the phenyl ring being optionally substituted by halogen, alkyl, alkoxy, hydroxyl or by a group --NR.sub.a R.sub.b, or --CONR.sub.a R.sub.b, wherein R.sub.a and R.sub.b are H, alkyl, alkenyl, or with the nitrogen atom form a saturated monocyclic ring;R.sub.3 and R.sub.4 are H, alkyl or propenyl or together form an aralkylidene group;Alk represents a Cf.sub.2-3 alkyl chain optionally substituted by one or two alkyl groups; andA.sup.1 represents a C.sub.2-5 alkenyl chain and salts and solvates thereof.The compounds have selective vasoconstrictor activity and are useful in treating and/or preventing pain resulting from dilatation of the cranial vasculature, particularly migraine. The compounds may be formulated in conventional manner with pharmaceutically acceptable carriers or excipients.