The invention relates to compounds of the general formula ##STR1## and physiologically acceptable salts and N-oxides, hydrates and bioprecursors thereof, in which R.sub.1 and R.sub.2 which may be the same or different each represent hydrogen, C.sub.1-8 alkyl, C.sub.4-8 cycloalkyl, C.sub.3-6 alkenyl, aralkyl with 1 to 4 carbon atoms in the alkyl residue or C.sub.1-8 alkyl interrupted by an oxygen atom or a group ##STR2## in which R.sub.5 represents hydrogen or C.sub.1-8 alkyl, or R.sub.1 and R.sub.2 may together with the nitrogen atom to which they are attached, form a saturated monocyclic 5 to 7 membered heterocyclic ring which may additionally contain the heterofunction O or ##STR3## R.sub.3 represents straight or branched chain C.sub.1-8 alkyl, alkoxyalkyl with 1 to 8 carbon atoms in each alkyl residue, C.sub.1-8 hydroxyalkyl, C.sub.1-8 alkoxycarbonyl, alkyl thioalkyl with 1 to 8 carbon atoms in each alkyl residue, halogen or aryl;R.sub.4 represents hydrogen, C.sub.1-8 alkyl, C.sub.3-6 alkenyl or alkoxyalkyl with 1 to 8 carbon atoms in each alkyl residue;X represents --O-- or --S--;Y represents .dbd.S, .dbd.O, .dbd.NR.sub.6 or .dbd.CHNO.sub.2 where R.sub.6 represents hydrogen, nitro, cyano, C.sub.1-8 alkyl, aryl, C.sub.1-8 alkylsulphonyl or arylsulphonyl;m represents an integer from 2 to 4 inclusive; andn represents an integer which is 1 or 2, or additionally when X is --S-- n may also be zero.The invention also relates to processes for the production of such compounds, pharmaceutical compositions containing them and certain novel intermediates used in their production, i.e. amines of the formula ##STR4## and alcohols of the formula ##STR5## The compounds of formula (I) show pharmacological activity as selective histamine H.sub.2 -antagonists.