Compounds of formula (I), or a pharmaceutically acceptable derivative thereof, wherein R.sup.1 and R.sup.3 independently represent H, C.sub.1-6alkyl, C.sub.3-8cycloalkyl, or halogen; R.sup.2 represents C.sub.1-6alkyl, CF.sub.3 or aryl; a represents 1 or 2; R.sup.4, R.sup.5, R.sup.7 and R.sup.8 independently represent H, C.sub.1-6alkyl, C.sub.1-6alkyloxy, CN or halogen, or R.sup.4 and R.sup.5, or R.sup.7 and R.sup.8, together with the ring to which they are attached form an aryl or heterocyclic fused ring system; X represents C or N; Y represents CH.sub.2 or O; R.sup.6 represents H, CN or halo provided that, when X represents N, R.sup.6 is absent. The compounds are useful for treating endometriosis, uterine fibroids (leiomyomata), menorrhagia, adenomyosis, primary and secondary dysmenorrhoea (including symptoms of dyspareunia, dyschexia and chronic pelvic pain), or chronic pelvic pain syndrome.