Abstract: Compounds of formula (I) and pharmaceutically acceptable salt thereof are useful as histamine H.sub.3 receptor ligands. R.sup.1 is an optional substituent such as C.sub.1 to C.sub.6 alkyl. The moiety (1) replaces any available hydrogen atom on a carbon or nitrogen atom in the ring which includes X. R.sup.2 is C.sub.1 to C.sub.8 hydrocarbylene (in which one or more hydrogen atoms may be replaced by halogen atoms, and up to 2 carbon atoms may be replaced by oxygen, nitrogen or sulfur atoms); R.sup.3 replaces any available hydrogen atom on a carbon or nitrogen atom in the ring which includes X, and is hydrogen or C.sub.1 to C.sub.15 hydrocarbyl (in which one or more hydrogen atoms may be replaced by halogen atoms, and up to 3 carbon atoms may be replaced by oxygen, nitrogen or sulfur atoms); the (or each) R.sup.4 group is independently selected from H, non-aromatic C.sub.1 to C.sub.6 hydrocarbyl, and aryl (C.sub.1 to C.sub.3 alkyl); X is --SO-- or --SO.sub.2 --; Y and Z are each hydrogen, or together represent .