A class of piperazinones, homopiperazinones and thione analogues thereof, substituted at the 1-position by an optionally substituted alkenyl, alkynyl, aryl-alkyl or heteroaryl-alkyl moiety, and linked at the 4-position via an alkylene spacer to a fused bicyclic heteroaromatic moiety, typically indolyl, are selective agonists of 5-HT.sub.1 -like receptors, being potent agonists of the human 5-HT.sub.1D.alpha. receptor subtype while possessing at least a 10-fold selective affinity for the 5-HT.sub.1D.alpha. receptor subtype relative to the 5-HT.sub.1D.beta. subtype; they are therefore useful in the treatment and/or prevention of clinical conditions, in particular migraine and associated disorders, for which a subtype-selective agonist of 5-HT.sub.1D receptors is indicated, while eliciting fewer side-effects, notably adverse cardiovascular events, than those associated with non-subtype-selective 5-HT.sub.1D receptor agonists.