The invention concerns a heterocyclic derivative of the formula I, ##STR1## wherein Ar.sup.1 is optionally substituted phenyl or naphthyl; A.sup.1 is (1-6C)alkylene, (3-6C)alkenylene, (3-6C)alkynylene or cyclo-(3-6C)alkylene; Ar.sup.2 is optionally substituted phenylene, or a 6 membered heterocyclene moiety containing up to three nitrogen atoms; R.sup.1 is hydrogen, (1-6C)alkyl, (3-6C)alkenyl, (3-6C)alkynyl, cyano-(1-4C)alkyl or (2-4C)alkanoyl, or optionally substituted benzoyl; and R.sup.2 and R.sup.3 together form a group of the formula --A.sup.2 --X--A.sup.3 -- wherein each of A.sup.2 and A.sup.3 is (1-4C)alkylene and X is oxy, thio, sulphinyl, sulphonyl or imino; or a pharmaceutically-acceptable salt thereof. The compounds of the invention are inhibitors of the enzyme 5-lipoxygenase.